My car has a wiring loom with empty sockets where the built-in GPS, cruise control and electric seat go if I had bought the more expensive version. If I were to stick these gadgets in, they would work just as well as if the factory had fitted them.
Note how in male/female children the nipples are the same. We humans have a lot of male/female bits in common: arms, legs, liver, stomach, etc. The nipples are a case in point. They are there if the female hormone kicks in. If it doesn't, then like my empty sockets, nothing happens. If it does then there are the rudiments there to develop.
If you think about it, it would be more complicated for men to not have nipples.