The purpose is to widen the opening of your vagina and make more room for the baby to pass through. But just because you had one with your first, doesn't mean you'll need one this time, in fact it's more likely that you won't.
Episiotomies are much more common with first-time moms.
They're also more likely to be performed when forceps are used for the delivery , or when the baby's breech or are very large. During the pushing stage of labor, the vagina and perineum stretch to make room for the baby. When the baby's head is crowning, if there's not enough room, the tissues will naturally tear. In most cases the tear is toward the anus, but it can also occur in the labia or toward the urethra. Episiotomies are used to expedite delivery when there's not enough time to let things stretch on their own, or to avoid serious tearing.