Big Dog, 5 (or 6) sets is one of many methods used to shock muscles into growth. Done a la BFL it is called a "Pyramid" and the first two sets are warm ups. There are other methods that utilize 5 sets, 5x5, BodyRx's Fat burning cycle, and then there's German Volume Training where you do 10 sets of 10 reps, to name but a few.

It's all about varying your lifting so that you and your muscles don't become bored and can continue to grow.

Nat