A radiused shelf, typically accompanied by a radiused arrow plate area on the riser does a couple things, beside looking nicer (to me)than a flat shelf.
It allows the arrow to make contact with only a very limited area of the shelf, and it also creates a definate launch point typically just above the deepest recess of the hand grip. It does do something, but, like everything else, if you like it flat, or if you shoot just fine with a flat surface, then it really doesn't matter.
ChuckC