Why covering up modesty is Wajib

'Enjoin the believing women to lower their gaze and guard heir modesty;not to display their beauty and ornaments except what normally appears thereof;let them draw their veils over their bosoms and not display their charms except to their husbands,their fathers,their fathers in law,their own sons,their stepsons,their own brothers,their nephews on either brothers' or sisters' sides,their own womenfolk,their own slaves,male attendants who lack sexual desires or small children who have no carnal knowledge of women.Also enjoin them not to strike their feet(on the ground) in order to draw attention to their hidden trinkets.And O believers!Turn to ALLAH in repentance,all of you,about your past mistakes,so that you may attain salvation.'Surah An-Nur;Ayat 31.