Why do Au Ra females have boobs?

[Cilia]

Not all men have an interest in female breasts. And I'm pretty sure most every species of mammal is capable of developing enlarged mammary glands. Apes, bovines, cats, dogs, etc.

Not all, but developed breasts are an indicator of female fertility, biologically speaking. (They develop during puberty, after all.) Thus, a female with visibly developed breasts is at least capable of producing offspring, even if they can't do so at this exact moment, and males will pay attention to her as a potential candidate for mating. (Yes, I am being cold and scientific about this. When such a question is asked, it requires that kind of attitude to be taken seriously.)

Another hypothesis for why human females have permanently developed breasts is Prison School's "boobs are just fake asses" stance. In honesty, though, there's no clear scientific reason to explain it.

While all mammals are capable of producing milk to nurse their offspring, humans are the only species that have permanently enlarged breasts. The breasts of other mammals do enlarge during pregnancy and while nursing, but otherwise they are not much different from a male in that regard. A human female, on the other hand, has enlarged breasts her entire adult life.