re why are folk buying super expensive ones?
Not sure. A proper roof box is watertight (ours stays out in the side passage all year and when we open it come spring it is still dry as a bone inside). So that is useful if you want to store things in it while you camp.
Men people have concerns about aerodynamicity and altered "handling" with shoving things up on the roof so I guess a plastic roof box with nose cone reduces this (saying that i did notice one car last weekend where the utter dumbos had put it on back to front).
You can cram a load of crap in them, annoying fiddly crap like camping chairs and the sod of a packing cobb.