I assume it's to give some shape or because it's expected. I'm a C cup and hate wearing a bra. I do wear one when I go out just to keep the nips in check and so I won't get nasty comments from other women. If I had As I would probably just use pasties if I was too worried about it, but the expectation of other women is really influential. They can be nasty, especially if you happen to be with a dude.
Women make nasty comments on other womens' nips? That's a thing? Like random women? Or just women you know? Do the comments come from anywhere or like just in the workplace?
lol, I'm a guy and understand those comments come from all the above. Yes, even random strangers. Try breast feeding in semi public even with a towel and the women find them to yell at them.