1) The bra can (read: must) reduce the bouncing (yes also flatchested girls' breasts bounce), which helps preventing the breast tissue from damaging.
2) The nipples might not show through their shirt when it's cold or whatever reason there would be causing them to be able to see through the shirt.
3) If you have small breasts a (well-fitting) bra gives a nice shape to your breasts. (And if you wear a push up it can make them look bigger and give you more cleavage).
By the way:
'A bra can prevent your breasts from sagging' or 'If you don't wear a bra your breasts will sag'.
This is not true. Actually, a bra should be avoided as much as possible (yes you should wear one when you go outside, but for example when you go to bed you shouldn't wear a heavily padded bra or something), because the bra takes over the function of your pecs, causing them to weaken. Result: your breasts start to sag.
Yeah, due to gravity bigger breasts will (read: have the possibility to) sag more easily, but genetics and the condition of your pecs play a big role as well.
so their nipples don't show
For shaping reasons. I personally hardly ever wear a bra, love the free feeling of not having one on.
Bras are flattering to the shape and size of breasts and can make them look bigger or perkier. (It's a myth that small breasts never sag.) They help them look good under clothes. Clothes that are popular today are geared towards wearing a bra and the shape of breasts without a bra can look odd under them.They help conceal the nipples. Also it's just the norm for girls who are past puberty to wear bras and not wearing one is sometimes seen as weird or indecent despite the girl's size.
Well even though small breasts aren't heavy or hurting the back, I wear bras cause it's shapes them up, making them look better and bigger sometimes lol. Plus I wouldn't want my nipples showing through my clothes =P
I don't know. I never wore a bra until I started developing. I would assume that it's so their nipples don't rub against their shirts and so that their erect nipples don't show as much when it gets cold outside. Also, even small breasts get droopy eventually, so it's understandable that they would want to wear bras.
Women wear bras for all the unnecessary reasons. They are as followed:
1) To deceive others(Namely men) into thinking that their breasts are larger than they actually are.
2) For artificial support
3) Too lazy to work out link
5) They conform to society's ideology: it's only moral to wear bras because breasts are now for some reason perceived as taboo.
My ex use to wear one for two reasons, so if she got aroused her nipples wouldn't show through her shirt and to give her a bigger look up top.