Thanks Sarge.
Backoff sets are really on useful for adding size. Basically, heavy low rep sets generate lots of tension, but the weight is too heavy for you to get deep into fatigue (although 5 is a pretty good balance). So, you have great strength potential working in low, heavy sets . . .but size tends to develop faster with higher reps, when you can generate a lot of metabolic fatigue, which causes some interesting stuff to happen. I won't pretend to know how to accurately explain it all but it involves hormone release, protein breakdown, lactic acid, etc. The backoff sets really just add local muscular fatigue into your workout. They're disposable really if you're just trying to build your 5s. Higher reps just adds in some more size-oriented work.