Like the male nipple, the female orgasm exists because it is so necessary in one sex that it appears in the other. Biological traits often come into being as a result of natural selection: those traits that help an organism to survive and reproduce get passed along to the offspring, increasing the population of those with that trait. This is called an adaptive trait. The female orgasm is thought to be an adaptive trait, either because the cervical contractions draw sperm into the uterus, increasing likelihood of fertilization, or because it increases the incentive for two people to have sex, thus increasing the probability of engendering offspring.
Read the entire article here: Why Women Orgasm: Do We Really Know?