It's probably one of those things that was believed to be necessary at the time, is still believed to be so by many OB's, but research has since shown to be completely unnecessary -- like the VAST MAJORITY of episiotomies.
I was given an episiotomy for DS. He wasn't even that big, under 7lbs, normal presentation... he was just 'stuck' in the birth canal for a long time -- I blame the drugs they gave me (to help me rest before the actual birth and were supposed to be worn off) so I wasn't 'all there' or focused to push well, and the fact that they had me side-lying. Anyway, he cut me, and I *still* had 4th degree tears.
But anyway, yeah, my understanding is that episiotomy was quite routine for just about ANY kind of birth, it was believed it would reduce the incidence of tearing. (It didn't).