The idea of the flat washer in this particular case is how the non-reinforced hole tends to concave and then cause sealing issues from overtightening. Same reason we put reinforcing plates on valve covers. This principle works for most thin sheet metal on the engine. The oil pan also benefits from a flat washer on each of the 18 bolts for the same reason. Since we use gasket sealer on the gasket and the bolts, there is very little worry about them backing out.
