There is two different types, one is like a silicone cover that protects it and provides a bit of grip, the other is a cushioned type that fits over.
Most of the reviews say that as well as protecting it, it gives extra padding on your hands/arms during movements like yoga etc and it's not so harsh under your feet..I think because the Wii board isn't flat.. kinda bumpy
I'll got for the silicone one then.... those battery packs, did you get that game? The reviews the models in Amazon are terrible. I do prefer to get Nintendo branded accessories