Niqab is an Arabic word, which is translated as veil in English but it is actually a mask used for face covering by Women in Islam.
Muslim women in the west today are rediscovering the pristine Islam as revealed by Allah, (the God), to the Prophet Muhammadﷺmore than 1400 years ago. Consequently they are essentially engaging in a life-long exercise of rediscovering their ownselves, what it means to be a human, a Muslim, and more so, a Muslim woman. Wearing a head-covering (hijab) is an important part of their spiritual journey.
One of the most common questions today, asked non-Muslims is: Why do Muslim women cover their heads and body? The answer is very simple – Muslim women observe hijab because Allah has told them to do so:
O Prophet, tell your wives and daughters and the believing women to draw their outer garments around them (when they go out or are among men). That is better in order that they may be known (to be Muslims) and not annoyed…. Quran Surah Ahzab 33:59
Muslims believe that their sole purpose in life is the worship of God alone, according to His instructions, as revealed in the Quran, and through the teachings of the Prophet Muhammadﷺ. As such, wearing the hijab is an act of obedience to God and, hence, forms the primary basis for wearing it. 
The following examples from history amply illustrate the fact that the status of women in earlier civilizations was very low to the extent that they were denied basic human dignity:
a. Babylonian Civilization:
The women were degraded and were denied all rights under the Babylonian law. If a man murdered a woman, instead of him being punished, his wife was put to death.
b. Greek Civilization:
Greek Civilization is considered the most glorious of all ancient civilizations. Under this very ‘glorious’ system, women were deprived of all rights and were looked down upon. In Greek mythology, an ‘imaginary woman’ called ‘Pandora’ is the root cause of misfortune of human beings. The Greeks considered women to be subhuman and inferior to men. Though chastity of women was precious, and women were held in high esteem, the Greeks were later overwhelmed by ego and sexual perversions. Prostitution became a regular practice amongst all classes of Greek society.
c. Roman Civilization:
When Roman Civilization was at the zenith of its ‘glory’, a man even had the right to take the life of his wife. Prostitution and nudity were common amongst the Romans.
d. Egyptian Civilization:
The Egyptian considered women evil and as a sign of a devil.
e. Pre-Islamic Arabia:
Before Islam spread in Arabia, the Arabs looked down upon women and very often when a female child was born, she was buried alive.
Islam uplifted women and gave them equality and expects them to maintain their status.
Islam uplifted the status of women and granted them their just rights 1400 years ago. Islam expects women to maintain their status.
People usually only discuss ‘hijab’ in the context of women. However, in the Glorious Qur’an, Allah first mentions ‘hijab’ for men before ‘hijab’ for the women. The Qur’an mentions in Surah Nur: “Say to the believing men that they should lower their gaze and guard their modesty: that will make for greater purity for them: and Allah is well acquainted with all that they do.”
The moment a man looks at a woman and if any brazen or unashamed thought comes to his mind, he should lower his gaze.
The next ayath of Surah Noor, says: “ And say to the believing women that they should lower their gaze and guard their modesty; that they should not display their beauty and ornaments except what (must ordinarily) appear thereof; that they should draw veils over their bosoms and not display their beauty except to their husbands, their fathers, their husbands’ fathers, their sons...”
The reason why Hijab is prescribed for women is mentioned in the Qur’an in the following verses of Surah Al-Ahzab: “O Prophet! Tell thy wives and daughters, and the believing women
The Qur’an says that Hijab has been prescribed for the women so that they are recognized as modest women and this will also prevent them from being molested. 
For more read: liberation of women through Islam
Allaah says (interpretation of the meaning): “And tell the believing women to lower their gaze (from looking at forbidden things), and protect their private parts (from illegal sexual acts) and not to show off their adornment except only that which is apparent (like both eyes for necessity to see the way, or outer palms of hands or one eye or dress like veil, gloves, headcover, apron), and to draw their veils all over Juyoobihinna (i.e. their bodies, faces, necks and bosoms) and not to reveal their adornment except to their husbands, or their fathers, or their husband’s fathers, or their sons, or their husband’s sons, or their brothers or their brother’s sons, or their sister’s sons, or their (Muslim) women (i.e. their sisters in Islam), or the (female) slaves whom their right hands possess, or old male servants who lack vigour, or small children who have no sense of feminine sex. And let them not stamp their feet so as to reveal what they hide of their adornment. And all of you beg Allaah to forgive you all, O believers, that you may be successful”Quran Surah Noor 24:31
Allaah commands the believing women to guard their chastity, and the command to guard their chastity also a command to follow all the means of doing that. No rational person would doubt that one of the means of doing so is covering the face, because uncovering it causes people to look at it and enjoy its beauty, and thence to initiate contact. The Messenger of Allaah ﷺsaid: “The eyes commit zina and their zina is by looking…” then he said, “… and the private part confirms that or denies it.” Sahih al-Bukhaari, 6612; Muslim, 2657. If covering the face is one of the means of guarding one’s chastity, then it is enjoined.
Undoubtedly the face is the site of beauty and attraction, so concealing it is obligatory lest men who do feel desire be attracted and tempted by her.
The Prophet ﷺsaid: “When any one of you proposes marriage to a woman, there is no sin on him if he looks at her, rather he should look at her for the purpose of proposing marriage even if she is unaware.” Narrated by Ahmad. The author ofMajma’ al-Zawaa’id said: its men are the men of saheeh.
The evidence here is the fact that the Prophet ﷺsaid there is no sin on the man who is proposing marriage, subject to the condition that his looking be for the purpose of proposing marriage. This indicates that the one who is not proposing marriage is sinning if he looks at a non-mahram woman in ordinary circumstances, as is the one who is proposing marriage if he looks for any purpose other than proposing marriage, such as for the purpose of enjoyment etc.
If it is said that the hadeeth does not clearly state what is being looked at, and it may mean looking at the chest etc, the response is that the man who is proposing marriage looks at the face because it is the focus for the one who is seeking beauty, without a doubt.
Those negative consequences include:
1 – Fitnah (temptation). By unveiling her face, a woman may be tempted to do things to make her face look more beautiful. This is one of the greatest causes of evil and corruption.
2 – Taking away haya’ (modesty, shyness) from women, which is part of faith and of a woman’s nature (fitrah). Women are examples of modesty, as it was said, “more shy than a virgin in her seclusion.” Taking away a woman’s modesty detracts from her faith and the natural inclination with which she was created.
3 – Men may be tempted by her, especially if she is beautiful and she flirts, laughs and jokes, as happens in the case of many of those who are unveiled. The Shaytaan flows through the son of Adam like blood.
4 – Mixing of men and women. If a woman thinks that she is equal with men in uncovering her face and going around unveiled, she will not be modest and will not feel too shy to mix with men. This leads to a great deal of fitnah (temptation) and widespread corruption. from Hamzah ibn Abi Usayd from his father that he heard the Messenger of Allaah ﷺsay, when he was coming out of the mosque and he saw men mingling with women in the street; the Messenger of Allaah ﷺsaid to the women, “Drawback, and do not walk in the middle of the road; keep to the sides of the road.” Then the women used to keep so close to the walls that their garments would catch on the walls because they kept so close to them. Tirmidhi narrated 5272, Classed as hasan by al-Albaani in Saheeh al-Jaami’, 929
Adapted from the words of Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him) in Risaalat al-Hijaab. 
Refutation For those who claim niqaab is not wajib and the face and hands of a woman can be seen by (ghair Maharem) strange men!
Refutation from Shaikh Ibn Uthaymeen
This is taken from the book "Hijaab" by Shaikh Ibn Uthaymeen from Saudi Arabia. Printed by Madrasah Arabia Islamia Azaadville- South Africa.
Translated by Hafedh Zaheer Essack, Rajab 1416 (December 1995)
The Ulamah who are of the opinion that it is permissible to look at the face and hands of a strange woman (who is not mahrrum) say so mainly for the following reasons.
The hadeeth of Ayeshah (Radhiallaahu Ánha) when Asmaa (Radhiallaahu Ánha) the daughter of Abu Bakr came to the Rasulullaah (Sallallaahu Álayhi Wasallam) while wearing thin clothing. He approached her and said: 'O Asmaa! When a girl reaches the menstrual age, it is not proper that anything should remain exposed except this and this. He pointed to the face and hands. But this hadeeth is WEAK because of 2 main weaknesses.
There is no link between Ayeshah (Radhiallaahu Ánha) and Khalid bin Dareek, who narrated the hadith from her. And in every chain of narrators Khalid bin Dareek is mentioned.
In the chain of narrators Sa'eed bin Basheer appears, who is known by most of the Muhaditheen as being a weak narrator.
This has been mentioned by Imaam Ahmad bin Hanbal (Rahimahullah), An-Nasai (Rahimahullah), Ibn Madeeni (Rahimahullah) and Ibn Ma'een (Rahimahullah). This is also why Imaam Bukhari (Rahimahullah) and Muslim (Rahimahullah) did not except this hadeeth to be in their books. (From Shaikh Ibn Uthaymeen in the book "Hijaab" pages # 17 and 18.)
We also have to see that the Muhadith Abu Dawood when he quoted this hadeeth put with it that it is Mursal (with a broken chain that does not lead up to the Sahabah). (From The Book "Hijaab wa Safur" under the fatwaa of Shaikh Abdul Aziz Bin Bazz on Page #61. Also stated as being weak by Shaikh Nasiruddeen Al-Albaani in his Daeef Sunan Abu Dawud in Kitab-ul-Libas under hadeeth number 4092 (which is the original hadeeth number)
An other thing that shows the weakness of this hadith is that after the ayah for hijab (Surah Al-Ahzaab – Verse #59) was revealed then the women of Sahaba wore a complete veil and covered the faces and hands. This includes Asmaa (Radhiallaahu Ánha) the daughter of Abu Bakr, who is supposed to have narrated this hadeeth. Asmaa (Radhiallaahu Ánha) covered herself completely including the face, this has been narrated in authentic hadeeth in Imaam Malik's "MUWATTA Book 20 Hadeeth # 20.5.16."