After birth, children receive milk from their mother’s breasts just as mammals do. A girl’s breasts are flat (just as a boy’s chest is) before puberty, but due to hormones released during puberty, the breasts start to develop. At this point the breasts develop mammary glands which can produce milk when the woman has a baby.
However, a woman’s breasts are much more than a milk factory. The breasts have a lot of sensitive nerves connected to parts of the brain that create sexual feelings. This is why most women like to have their breasts touched during sex.