The presence of nipples and other breast tissue in men exemplifies the fact that the basic body plan of men and women is similar.
There are exaggerations in each sex based on early development and later hormonal influence.
The nipple happens to be one of the features whose full development has been restricted in the male during early development so it never develops its full function, which is found in the female.
As far as is known, male breast tissue has no functional significance, although it is considered an erogenous zone in men as well as in women.
But the tissue is still there, and actually can respond to female hormones, as exemplified by the phenomenon of gynecomastia, or abnormal enlargement of the breasts in men.
This is seen in conditions in which there is excess estrogen function in the male.