It keeps them from flopping around when walking or running. And just because her nipples are showing, doesnt mean shes not wearin a bra. She could be wearing a thin bra, or her nipples are big and hard haha
But yeah. Then theres the pushup bra
haha which brings the boobs up, giving more cleavage. Then theres a sports bra thats REALLY tight so when she runs, her boobs dont go all over the place and hurt her chest.