Well I like to bench run a engine for a little while to dial in the carb some. It can be a lot easier that way, than trying to do it in a car at first.
But I have run them in cars too, so it could go either way. Years ago when i was a lot more interested in cars we used to have to run them in the cars as at the time we couldn't put a prop on them. But sometimes it was a pain getting a cantankerous  engine to run and dial it in too.