With the beam width of the radar scanner it is advisable to raise it on a bracket just so people's heads are not scanned on deck. They emit microwaves at reasonably highpower, I am not fond of fishing with lead foil wrapped round my head.
Another reason is less obvious, the housing for the electrics needs a completely flat surface for mounting. most cabin roofs are convex, if you tighten the casing down onto it you can crack the casing and that will allow water to get in. The electrics will not last long if damp gets in there never minda crack letting in water.