[Brian]

Grease allows surfaces to slide over each other as components are torqued up. Seals any microscopic voids/ imperfections. Any grease that exudes into either the valley or intake runners is assimilated into the engine with no harmful effects. Also, prevents the gasket from sticking to any surfaces, allowing easy subsequent removal, and indeed, as Ol Ron says, you can even reuse the same gasket after applying another layer of grease.