I think that breast serve a concrete purpose that is essential for our society. And the fact that we have breasts is strongly tied in with the difference between us and other animals. We are different from other animals in that we are more clever and more social. We are also different in that we have breasts. It's likely that the two are tied together.
My theory is this: breasts are an age indicator. In traditional human societies it's difficult to judge the reproductive health of a woman. A woman can hide her true reproductive health by various means (like body painting or ovulating on the sly), so as our societies grew bigger and bigger, it was neccessary for the society as a group to make non-hideable means of judging how fit the particular woman is.
Breasts benefit the community more than they benefit the woman according to my theory. I.e, humans are supremely social, and the best evolved human is the one that works the best to move the entire community forward. A society of very selfish people will not survive.
Humans have certain traits that help the community more than they help themselves. Breasts could be one of them.
Small, large, medium, the size of breasts is not really critical. However, perk and 'young' breasts would always be judged more attractive than long sagging breasts. It's clear that the function of breasts is correlated to age. And that sexual attractiveness increases the more attractive the breast is.
Similarly, if a young woman is starved for a long time in a sand blown deserts, the first things to go are the form of the breasts. She is not in an ideal reproductive state, as rearing children requires a lot of water. Old women who are past reproductive age often have their breasts change to a different less attractive form.
Conclusion: Breasts, and some other female parts are a simple indication of youth and reproductive health for a society that is clever enough to fake the other things usually used by other primates.