Importance of Hijab in Islam
Question

Is it right for someone to say that since wearing Hijab is not one of the five pillars of Islam therefore it is really not that important?
Praise be to Allah.This is a mistake, because Hijab is obligatory for women . Allah is the One Who has commanded women to cover their adornments , including the face, bosom and all their adornments. 
Hijab has been enjoined upon women as a protection against abuse and temptation. That is because women are the focus of desire, and the focus of what those who are tempted look at.
So if a woman uncovers her beauty she will provoke desire, and people will become attracted to her and will follow her. This is the cause of many immoral actions such as adultery and the things that lead to it. So Hijab is obligatory for women and was enjoined upon them in the verse (interpretation of the meaning):
“…and to draw their veils all over Juyubihinna (i.e. their bodies, faces, necks and bosoms)…” [An-Nur 24:31]
The veil (Khimar ) is that which comes down from the head and covers the face, and the Jilbab is an outer garment with which a woman covers herself, leaving no part of her body showing.
And Allaah says (interpretation of the meaning): 
“…And when you ask (his [the Prophet’s] wives) for anything you want, ask them from behind a screen…” [Al-Ahzab 33:53] 
So Hijab is a protection for women so that there will be no room for people to treat them as playthings.
And Allah knows best.