More pertinent would be to ask why Troll girls HAVE boobs or even why there are females, considering the fact that they breed through the Mother Grub and the mixture between the stuff from either sex can put in buckets seems to be compatible regardless of gender. The breasts could just be secondary sexual traits (like the big tail feathers of peacocks), but that would just be guessing. (ARQuius suggests female trolls don't make milk, so that's out) Perhaps the Mother Grub wasn't always involved

It’s never been explained in canon but a theory I enjoy is that females are the more aggressive sex in trolls and the breasts are extra padding acting kinda like armor allowing them to take more hits. This is supported by several female trolls having aggressive tendencies as well as Flarp, a very dangerous game being called a “game for girls” by Karkat.