A RTT is for two primary reasons.
1. Ease of set-up and break-down. They fold-up with the bedding all in place. I could set-up my 1200 Eezi-Awn in about 30 seconds
2. Use in any terrain, ground condition or weather. Nasty, muddy, rocky ground is not a constraint to the RTT.
Of course, there are downsides, like cost and weight. I am not a proponent of the RTT on the roof, though it is acceptable if used with a light cross bar and smaller tent. About 140 lbs. max. At trailer or bed height is better, as with my old Tacoma or my Chaser trailer. I do not have a RTT on my Land Rover, and am happy using a quality ground tent.
For the Overland JK, we are testing a few tents on the roof. This truck is wide, long and stable and when combined with the good suspension, the weight is less noticeable.