Think of the episiotomy this way: if you hold a piece of cloth at two corners and attempt to tear it by pulling at the two ends, it will rarely rip. However, if a small cut is made in the center, then pulling at the ends easily rips the cloth. Doing an episiotomy is analogous, and sometimes results in tears that extend into the rectum. Physicians argue that this "clean" tear is more easily repaired than the ragged one that occurs when a woman tears without the cut. My experience has been that the small tears that sometimes occur without episiotomy are easy to stitch and less bothersome to the woman. Episiotomies, once repaired, are often debilitating and are the source of much pain in the postpartum period.
-Harrison 1982:97