Female cats need nipples for nursing litters of kittens. Male cats don't really need nipples at all but, just like a man has nipples, so does a male cat. Since he doesn't need them, why does your boy kitty have teats?
The mammary system refers to the system of mammary glands and teats present in all mammals. It consists of glands that in females can produce milk when she is pregnant and nursing. Each gland is covered by skin and fur, and you can tell the location of these glands because of the visible teat or nipple.
Finding His Nipples
Finding the nipples on your kitty can be tricky. They're covered by his tummy fur, and you'll have to lay him on his back and comb back his fur to see them. He'll have four pairs of nipples, lined up in two rows on his stomach. In males, the nipples look just like a female's, but they don't have the ability to produce milk.
In a female cat, the teats are essential for nursing her litter of kittens. Each teat has a unique smell so a kitten can return again and again to same nipple to feed. They have an extensive blood supply. When her hormones change due to pregnancy, it will signal the mammary glands to begin producing milk. She'll produce milk not only while she's nursing, but up to 50 days after she's weaned her kittens.
Why Do Males Have Nipples?
Since males don't use their nipples for nursing little ones, why do they still have them? In males the nipples are vestigial, there but with no real function -- just like the remnants of hip bones in whales and vestigial tail bones in humans. Even though they don't use them, evolution hasn't made them completely disappear. While having breast tissue does make them able to get some of the same diseases that affect females, like breast cancer, this is extremely rare and not enough of a reason for the nipples to completely disappear. Evolution hasn't proven them to beneficial, but they continue to exist because there haven't been any real negatives to having vestigial nipples. Since both males and females possess the genes for nipples, they both have them.