No reason other the macho/ego idea of hearing their engine run (even more somwith the diesel guys)!
As far as cooling down the turbo......by the time you get to the campground and drive to the campsite it's already cooled down. In he morning same thing, by the time you get to the main road it's warmed up. It does need to idle a bit to fill the air tanks and suspension.
With that said, I was in a gas station one time and watched a lifted diesel truck pull in from t of the store, idle up the straight pipe exhaust and go inside for 10 minutes! Oh ya, it was 40 degrees outside! Thentruck was new enough that it wasn't a hard starting diesel and could've been shut down altogether. Some people just don't know much at all!