so they can charge twice as much to change the spark plugs at the dealership haha.

no im not sure the exact reason but i think the second plug burns the excess fuel thats left after the first spark and helps with emissions.

 

I think you're right. More complete burn, and better Fuel Efficiency.

 

x100 on the AC Delco plug suggestion. Don't get those E3's you see advertised on the forum. We have nothing to do with them, they're just an advertiser with the people who own the forum. I've heard bad things about those E3's though. They won't last you near as long as AC Delco's.
THere has been plenty of discussion about what type of plugs work best in the 4.3's. Just do a quick search on the forum and you'll find lots of info.