Well firstly it takes me about 5 min to unpack or pack up, and I can stop pretty much anywhere where there is some level ground. Dont have to worry about finding smooth soft area to pitch up the tent.

Secondly it is very secure on top of the vehicle, dont have to worry about anchoring it in strong wind.

Thirdly when you are in a national park deep in the bush it is comforting to know that some elephant, lion, hyena, etc is not going to just walk into or over you while you sleep.

Lastly when you park next to a river or the sea, it is always great to wake up in the morning and have an extended view of what is around you.