Surprisingly to many, the premise isn't true. The theory is, all primates breast feed, but no others have breasts like those of human women. In other primates, they are mostly present on an as-needed basis. The amount of tissue necessary for actually lactating is very small. They are for feeding babies, of course, but they are also a signaling device, like the tail on a male peacock. Much of the superficial anatomy of people of both sexes is literally for attractiveness, both visually and otherwise. Not being covered in hair, for instance. Facial beauty. Long flowing hair on the head. None of it makes humans more fit physically to survive. It's all about reinforcing the strength of the social bonds necessary to keep offspring that are immature and dependent for fifteen years or more. The insanely over-elaborate human brain, in particular, language, is also believed to be as much about entertaining each other for mating purposes as it is about tool making and planning. The utility of a giant brain for practical tasks may have come about as a useful secondary purpose.