We all start out as the same sex genetically...but early on we differentiate. Still, the genders share a lot of physical parallels as a result. Men not only have nipples, but we also have breasts of course.
I've also read a number of accounts where men were able to lactate, either as juveniles (a phenomenon partially described as a process called galactorrhea) or as adult males.
It actually isn't used for anything on males.
"nipples aren't a sex-linked characteristic. In other words, nipples are just one of those sexually neutral pieces of equipment, like arms or brains, that humans get regardless of sex. "
Nipples are VESTIGIAL PARTS like the coccyx, and the hair on the body. They are a part of our evolutionary history. They aren't functional simply because men do not have female hormones, of course! Basically they're of no use, but they form a part of the human museum of vestigial organs.