No male hylian has visible nipples. It's possible that female Hylians do, but in the absence of evidence I'm hypothesizing that their breasts are simply objects of sexual attraction with no further practical purpose. This is possibly caused by convergent evolution with humans, who have in the past coexisted with hylians and possibly crossbred as well.
This raises the question of how hylians feed their young. I think the simplest answer here is that they regurgitate partially digested food like birds. Gross? Only to our human sensibilities. To a hylian, who see breasts as strictly sexual, breastfeeding may be similarly upsetting.
