Dull answer off elsewhere on internet below - if its a choice between men and women having nipples or men and women not having nipples then I'd go for both having nipples - breasts would be so much less interesting without a nipple to aim for
Essentially, men have nipples because nipples are part of the basic human design option at conception. While modern societies may view nipples as secondary sexual organs, in reality a fetus of either sex develops nipples within a few weeks of conception. The human fetus actually develops several sets of nipples, much like other mammals, but only one set will fully mature in the womb. At this point in a fetus' development, there is no genetic difference between male or female. All fetuses develop nipples, chest muscles and milk glands.
The situation does not change until the chromosomes which determine sexual characteristics have the opportunity to affect development. This is the classic dividing line between females with the XX chromosome and males with the XY chromosome. While the nipples themselves may appear on both male and female fetuses, it is the introduction of hormones such as estrogen and testosterone which determine whether or not those nipples and breast tissues will function or not.
There are very few scientific conclusions concerning the original purpose or function of male nipples. Some human body parts, such as the wisdom teeth or appendix, did serve a real purpose for early human civilizations but have since become redundant or vestigial. There is little evidence, however, that men ever fed their young from their own lactating breasts.