It's amazing how few know that you should wear a sports bra when you play tennis, for example. Did you ever hear a girl complain about boob pain or bra burns? That's because they don't wear a sports bra. But you know what else?
If you play sports a lot and use a regular bra, your boobs will become saggy. It's been proven scientifically. Sports bras prevent that and are highly effective for A Cups (A cups are the best cups) when it comes for shape, but also hold the big boobs in place (so the big boobed girls don't have to reduce their breasts to match up to the pretty small-breasted girls).
If you're fat, don't wear a sports bra - why waste one on you? Go swimming or some shit and come back when you're not a fat hog. Leave cool sports to pretty girls.