I do it for a couple of reasons:

1. I don't get to go enough to any particular lake to really get to know it well. I also don't have fancy electronics or a bunch of brushpiles marked. Spider rigging allows me to improve my odds of catching a few and helps me to learn the lake better.

2. I get carpul tunnel symptoms from holding a pole for longer than 10 or 15 min at a time. My hands/arms will go numb and if I push it to long I'll end up having trouble sleeping. Running a chain saw or weedeater does the same thing. Spider rigging or trolling means I can fish with out that worry.