There is one situation, really, that I know of where 4wd is almost essential. (I put a lot of miles on a 2wd before finally going 4wd.) Happened to me - going down a really steep, loose, rocky road at the bottom of which was a deep washout (estimated 8' deep by 5' wide) cutting the road that nothing with wheels was going to cross. No place to turn around. I had to back up that hill but steep and loose as it was I could not back up - both tires were spinning. I ended up loading about a half ton of rock in the back of the van over the rear tires and having to walk the road and kick off a lot of the loose rock to get enough traction to back out of there. And the automatic transmission didn't like that backing up part much either.
Either of my 4wd vans, in 4-Lo and with the front tires pushing, would have backed out of there with no problem.