I make custom jeans and most of the girls want something subtly different, without the jeans wearing them. That is, if you look, they're special, in lovely denim and with a small feature somewhere that doesn't shout at you. I have a knack or two for making their rear end look beautiful - that does go down well. Bums are in.
Honestly, that question could be different for everyone. Some do it because they like that style. Others do it to impress other girls so that they are accepted. Others try to dress to make guys stare at them. It's different for everyone.