There are a lot of topics on this already, I recommend a search here or on Mytabletennis for deeper discussions.
Short answer: a thin layer keeps the blade from splintering when changing rubbers. Very small changes to feel, almost none to performance.
Thick, multiple coatings, and different types of varnish can help harden the top layers of wood. Generally increases low impact speed and sharpens the impact feel slightly. But you need a lot of layers to make a noticeable difference.