Evidence suggests that female breasts have been regarded as a primary sexual characteristic for the majority of the time our species has been around. The obvious over-development (compared to most other species) suggests a) they were a key element in attracting a mate and b) that sexual encounters were probably face-to-face. This may be total rubbish, of course, as it was from something I read a long time ago, but it makes a kind of sense. There is no other reason I am aware of as to why human breasts are much bigger and more prominent than is necessary for their primary function, compared to say chimpanzees or gorillas. Or perhaps I need to get out more.