hy do guys have nipples and breasts if they don't breastfeed? Do they serve any useful function?

With very few exceptions, all mammals have breasts and nipples, whether they are male or female. Before humans reach puberty, the breasts of a male are basically identical to the breasts of a female. When a girl starts developing at puberty, her breasts continue to develop toward their final adult stage. Normally, the male breasts do not develop beyond their pre-pubertal stage.

Can a male have breasts during puberty?

It is very common for some males to have a minimal amount of breast development during puberty. It will usually be limited to a swelling under the nipple that may become the size of a small grape or a little more. It is often found on only one breast, and a fourteen year old male might have doubts about his eventual "masculinity". The swelling may be noticeable for up to a year and a half. If there are any concerns, ask your personal medical caregiver about it. Welcome to puberty.

What is gynecomastia?

"Gyneco" means 'pertaining to a woman' and "mastia" means 'breast'. A condition that may be caused by many things, including an imbalance of hormones caused by a tumor or some type of medication. Prescription medicine labels often warn of the possibility of this as a "side effect". It may occur on one side or it may be bilateral (on both sides). This is commonly seen in males during the start of puberty, but mention it to your personal medial caregiver.

Should men be aware of the dangers of breast cancer?

Absolutely! Nearly 1300 men in the United States are newly diagnosed with breast cancer each year, and about 300 die from it annually. While that is a small number, compared to the number of women that are diagnosed/die from breast cancer, it is large enough that men should be aware, and should seek advice as soon as they notice any changes in the breast. Men should do Breast Self Exams too. It is usually easier and faster for them then it is for their female partners. Remind each other so neither of you misses it each month.