Nipple direct the eye to the curve of the pectoral muscles. Without them the male chest would look very strange indeed.

Men and women are mammals (Latin for mamma breast) a species that has breasts and feeds its young with milk.

The human embryo contains tissue that evolves into breasts. Both male and female  babies are born with nipples and ducts intact. It is the presence of genes which directs hormones to differentiate the breast tissues. The Y chromosome stimulates testosterone and other hormones form male features. 

The nipple is actually the central protrusion inside of the pigmented areola. There is a thin muscle just under the skin that when stimulated, tends to flatten the puffed male nipple areola. This muscle when stimulated in a breast feeding woman, projects the nipple for the baby. Nerves that innervate the nipple / areola can produce pleasure for both men and women. The male chest without nipples has an unusual look.

Insufficient testosterone, hormone unbalance  and poor tissue responsiveness can all cause male breasts (Gynecomastia). An unbalance of hormones during male puberty that can result in some temporary breast growth, this resolves for most. There are also many medical problems and drugs that can cause male breast growth. 

Nowadays male breast enlargement (Gynecomastia), is a very common condition. It often results  in horrifying embarrassment and social distress. Many men try to hide man boobs with thick shirts, avoiding bare-chested activities, and withdrawing from public contact. For many the only solution was surgery. Now with Gynexin male breast reduction men can stop gynecomastia naturally.