I had one w/ my first baby. He was simply a bit too big for my body frame. It's better than having a c-section and that's what I almost had to do.
The baby being too big and the doctor trying to prevent actual natural tearing is the reason for the episiotomy.
Try not to worry. You very well may not need one.
thats even if one is necessary! having a bath during labour/water birth helps reduce chances of tearing. do not panic too much about it though!