A tow dolly is useful if you dont have a car that can be towed 4 down or have several front wheel drive cars that you might want to bring at different times. It can be much less expensive than setting a car up to tow 4 down . Its not that difficult to use but you do have a extra piece of equipment to deal with. Mostly Mine slides under my Coach and out of way.
I have Two cars i take at different times. Mine can be towed 4 down but has 200000 miles on it and is not worth setting up and my DW car is not towable 4 down. rather than lose thousands and buy a new car, a $1100 used dolly, all in, made sense for us. a A full 4 down set up new and installed inc braking system was over 3 grand.