There is actually a well supported (as in evidenciary support) hypothesis that the reason human females have protruding breast tissue is that it induces human males to stick around after the birth of children, whereby the infant human is more likely to survive by having protectors and providers available.
Basically, an evolutionary way of saying:
"Hey! If you stick around, there's toys to play with..."
After all, even though other mammals have breast tissue, including human males, none of them have the protrusions that human females have; even a cow, which has big teats (incidentally pronounced "tits"), has only what is needed for function.
The shape, varying sizes, and differing details are only found on humans, and the whole thing is only found on human females, and is very plainly esthetic... and esthetics are evolutionary for a reason.
Best guess, according to the paper I read, is that they look like this, because it will convince interested parties to continue association with the owner of nice boobies.
....
I probably sound like I'm listening to Marvin Gaye, and you're not wrong, but... I'm inclined to agree with the hypothesis...