They can be deployed relatively quickly, easily, on any terrain keeping occupants high and dry, and comfortable on-the go, usually with a built-in mattress. They ensure you'll have a comfortable sleeping platform when you don't know where you're going to end up at the end of the day.
Its all about convenience imo. I use a ground tent and air mattress that takes roughly an hour to 1) find a spot, 2) set up tent, 3) inflate air mattress. Of course it could take hours to find a suitable campsite if you don't know where you're going. You go with an RTT, and you 1) park, 2) unfold tent.
An RTT might be inconvenient if you're camping at a fixed location, and driving during the day - only to return to the same location each night. In which case you'd have to fold it up every day. Its a trade off, but one can get around it by putting the RTT on a camping trailer and have the best of both worlds.
Depends on what you do too - if you just go four-wheeling, then I don't think you'd want to lug around a RTT, nor raise your cog with that kind of thing. But if you do alot of overlanding, and cover alot of ground, an RTT makes all the sense in the world.