"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...
"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..." [Interpretation of the meaning: Al-Qur'an 24:31]
Hijaab includes conduct and behaviour among other things
Complete 'hijaab', besides the six criteria of clothing, also includes the moral conduct, behaviour, attitude and intention of the individual. A person only fulfilling the criteria of 'hijaab' of the clothes is observing 'hijaab' in a limited sense. 'Hijaab' of the clothes should be accompanied by 'hijaab' of the eyes, 'hijaab' of the heart, 'hijaab' of thought and 'hijaab' of intention. It also includes the way a person walks, the way a person talks, the way he behaves, etc.
Hijaab prevents molestation
The reason why Hijaab 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 that they should cast their outer garments over their persons (when abroad); that is most convenient, that they should be known (as such) and not molested. And Allah is Oft-Forgiving, Most Merciful." [Al-Qur'an 33:59]
The Qur'an says that Hijaab has been prescribed for the women so that they are recognized as modest women and this will also prevent them from being molested.