Good question. Why a woman has a nipple is something that is easy to understand - to feed her baby. We are mammals, and like other mammals, even rats and whales, we have nipples and hair. The nipple may be hard to find in some mammals and hair may be microscopic, but they certainly have it.
The exception to this provision is a platypus that does not have a nipple. Her baby licked the milk out of her mother's pores. Platypus can be ruled out, but how about a nipple to a man? This is because all humans start embryonic stages simultaneously.
If you get an X chromosome from both your parents, you will become a woman. If you get the X chromosome from your mother and Y chromosome from your father, then you will become a man. The genitalia (ovaries and testes) begin to develop from the fifth to the tenth week. But before that, everyone gets what's called a "milk bridge", two lines of tissue stretching from under the arm to your crotch area.
Slowly, this "milk bridge" disappears, leaving two nipples, although there are some who have three or more, along the former bridge. Nothing happens with human breasts until puberty, for women, but not men. The nipples in men (and all male mammals) have absolutely no function, just to remind us how we came from.
PS: Once born, baby girls and boys can remove a little liquid from their nipples, This liquid is sometimes said to be "witch milk"
