According to experts, men have nipples because it is something that occurs to each and every human being on the planet, regardless of gender. The nipples have been imprinted into our DNA blueprints the moment we were conceived in our mother’s womb. Thus, whether we get an XX or an XY for our twenty-third pair of chromosomes, we will always have nipples in our breasts.
Theories have also circulated that it could actually be possible that we were all conceived as bisexual beings. This means that we both had male and female reproductive organs during conception but, in time, the organs of the other sex would disappear gradually as we develop in the womb. However, this case is not fully supported because, as we can all see, male nipples are complete blood vessels and nerves that were present in fully developed organs as well.
Male nipples are also known as vestigial organs. At some point, looooooooooong ago, some purpose was served by them. However, since they are no longer needed or used (like the appendix, for instance, or the tail that everyone has prior to it being absorbed while still a fetus), they simply exist because they are still there.
I've seen a video explaining this. Which I'll post later because I'm currently at work. The youtube channel is ASAPScience. It states that we were all once ladies and something pointing to men having nipples because of our "lady side". It serves no real purpose though which is shown in another youtube video from Alltime 10's totally useless body parts video. But yeah. I'll be sure to get back to you on this later.
Actually in the womb, we're all girls until the testicles start to secrete male hormones, which make boys into boys. There is a disorder when this doesn't work called " testicular feminization syndrome". This person looks like a girl in every way except there is no internal uterus. She has undescended testicles in her abdomen, which don't function. She doesn't have any ovaries and never menstruates. She also does not have a cervix. From the exterior, one could not tell that this individual was supposed to be a boy, if only the hormones had worked properly.