The word "mustahabb" means "seeking the love (of Allah SWT)". A hadith qudsi beautifully describes the way to seek the love of Allah SWT:
Abu Huraira reported that the Prophet said, "Allah said: My servant draws not near to Me with anything more loved by Me than the religious duties I have enjoined upon him, and My servant continues to draw near to Me with supererogatory works so that I shall love him"
found in Sahih Bukhari
Our service of Allah SWT through our obedience of whatever He has commanded us draws us near to Him, and by doing more than has been commanded of us, we may draw nearer to Him still, inshallah.
Clearly, hijab is a command from Allah SWT and sisters draw near to Him through wearing hijab. And clearly, doing more than has been commanded is a way to draw still nearer to Allah SWT.
When a sister is already covering everything but her face and her hands, and she would like to do something extra to seek the love of Allah SWT, the only things left for her to cover are - her face and her hands!
Even if there were no other reason to wear niqab, surely this would be enough! How can it be "extreme" to wear niqab or gloves when her face and her hands are the only things the Muslim sister has uncovered in public to begin with??
Niqab as haya
The word "haya" is often translated into English as "shyness", but this does not really give a very good idea of its meaning in Arabic. A better, if longer, translation of its meaning might be "keeping private what should be private".
Islam places a very great importance on privacy, and on keeping private what should be private. Hijab is in fact part of a larger code of conduct, and constitutes only one aspect of haya.
Allah SWT has through the Shari’a clearly distinguished between public and private space and placed a screen (i.e., hijab) between them. Private space is physically divided from public space and strongly protected against any incursion (Surah an-Nur ayat 27-29). What people do in private space has also been protected and screened. This is accomplished by the bans on speculation, gossip and spying (Surah an-Nur ayat 12-13 and Surah al-Hujurat ayat 11-12). What is private must be kept private, by the physical protection of walls and by the conduct of Muslims in not talking about it, speculating over it, or looking into it. The same rule applies to the physical person, as what is not necessary to be displayed for some task should be covered (or, as the Quran says in Surah an-Nur ayah 31, women are "not to display their beauty except what is apparent of it"). Furthermore, in the private space of the home and family, rules are relaxed. People within the special group of family may visit freely (Surah an-Nur ayah 61) and be at ease in dress (Surah an-Nur ayah 31). By contrast, in public space, rules are strict. This includes not only dress but also conduct: physical contact should be avoided, talk should be business-like, and khulwa (i.e., an unrelated man and woman being alone together) should be avoided. In the dress, speech, and behaviour of the Muslim, there should be a screen which separates the public (that which is necessary to be made known) and the private (that which is not necessary to be made known).
From this, we can see that hijab is a screen of privacy, an act of haya. Clearly, it is mustahabb to screen our privacy even more than has been commanded. We can respect other peoples' privacy more carefully through avoiding speculation, gossip, and spying; and we can protect our own privacy more carefully through taking extra steps in modest dress, in avoiding physical contact and khulwa with non-mahrams, and in keeping conversation with non-mahrams to the minimum necessary to conduct our business. For sisters, as stated above, that extra degree in modest dress must and can only be niqab and gloves.
Niqab as taqwa
We can also look at another aspect. Sometimes people criticize Islam or Muslims for being too concerned with outward things and not concerned enough with inner things. It is true that this can lead to hypocrisy. Yet we should not be discouraged by hypocrites or let them push us to the opposite extreme of saying that outward things are not important at all.
Sometimes the outward things help us develop the inner, by making us more aware of Allah SWT. This awareness that Allah SWT is watching us is called in Arabic "taqwa".
Hijab can increase taqwa. When a sister sees her own reflection and her hijab, or when she becomes aware of it as she wears it, she may be reminded that she dresses like this because Allah SWT has ordered it, and because she knows that He is aware of what she does. These thoughts may inspire her to behave in the best possible manner.
So just think how much more of a reminder niqab is!
As well, for many sisters, hijab is a spiritual jihad because it often seems so difficult to wear. At times we may be forced to look deep into ourselves and find our faith and our courage. This provides many spiritual benefits in itself, and an increase of taqwa.
Again, just think how much more of a challenge niqab is, and how much greater the benefits when the nafs (inner self) has been conquered!
These are just some of the ways that niqab is mustahabb, some of the qualities and benefits of hijab that niqab is an extra degree of. Even if there were no record in the Shari'a of niqab, yet all of these reasons would still be true, and it would still be mustahabb to wear niqab.
Above are a number of different perspectives on why niqab is mustahabb. Niqab is just an extra degree of hijab and thus whatever benefits hijab brings as a commandment of Allah SWT, niqab brings an extra degree of by being a supererogatory (nafl) act.
But niqab is also SUNNA. That is, extra modesty in general is mustahabb, and niqab is the specific form of extra modesty indicated by the Shari'a.
How has niqab been indicated by the Shari'a?
1) For Ummahat al-Muminin (rAa), the wives of the Prophet (sAas), niqab is fard. It has been commanded in Surah al-Ahzab ayah 53 and the hadiths confirm that Ummahat al-Muminin (rAa) covered their faces in obedience to the command in this ayah to screen themselves from non-mahram men. (My note: If you claim that niqab "presents a bad image of Islam" or "is oppressive" - think! Would you say that if you saw Ummahat al-Muminin (rAa) wearing their niqabs? There is no dispute that Ummahat al-Muminin (rAa) wore niqab, and this fact alone makes it clear that niqab is part of Islam.) Ummahat al-Muminin (rAa) are also a model to all Muslim women and this is another reason in itself that niqab is mustahabb.
2) It appears that during a time in Madinah when the Muslims were being persecuted (as mentioned in Surah al-Ahzab ayat 57-61), it was fard for all Muslim women to draw their jilbabs over their faces. Subsequently, this understanding of Surah al-Ahzab ayah 59 was superceded by Surah an-Nur ayah 31, which allows the display of the face and hands. Please see What is the Final Rule on Hijab? for more about this point. As mentioned in Evidences for Jilbab, the purpose of the jilbab is to protect the Muslim woman and to assert her Islamic identity; clearly then, niqab is an extra form of protection and an extra step to take in asserting Islamic identity.
3) Even after Surah an-Nur ayah 31 had been revealed, ordinary Muslim women continued to wear niqab with the approval of the Prophet (sAas). This has specifically been mentioned for Umm Khallad (Sunan Abu Dawud Book 14 #2482), Asma bint Abu Bakr (Muwatta Book 20 #20.5.16), and some Qurayshi women who were visiting the Prophet (sAas) (Sahih Bukhari Book 54 #515). As well, the fact that the Prophet (sAas) had to tell women not to wear niqab and gloves in ihram (Sahih Bukhari Book 29 #64) means that niqab and gloves were well-known and worn by a substantial number of sahabiyat (rAa). Clearly this form of extra modesty has the approval of the Prophet (sAas) and that is another reason that it is sunna.
The descriptions of the dress of Ummahat al-Muminin (rAa), of the way that the jilbab was worn when Surah al-Ahzab ayah 59 was first revealed, and of the extra-modest dress worn by some of the sahabiyat (rAa) all clearly and unambiguously point to a face-cover. This makes niqab the specific form of extra-modest dress set by the Shari'a. As such, it is sunna as well as mustahabb.
|Hijab is a screen of privacy||Niqab is a better screen|
|Hijab helps develop taqwa||Niqab helps develop more taqwa|
|Hijab is a jihad that purifies the soul||Niqab is a greater jihad|
|Hijab is a protection for sisters||Niqab is a better protection|
|Hijab is an assertion of Islamic identity||Niqab is a stronger assertion|
|Hijab is fard||Niqab is mustahabb|