ho2zoo,
Wet sanding is just like it says. You sand using water to carry away the material you sand off. You need to use wet-dry sandpaper so that your sandpaper doesn't come apart from the water. The material is non-porous materials like metal. Wet sanding accomplishes two things. First it makes your paper last longer as the water carries away the sanded material so the paper doesn't clog up. Secondly, it provides a better finish because the material carried away by the water doesn't "scratch" the surface you are working. You wouldn't wet sand materials like wood because the water would warp the wood. Overall, when it's used wet sanding will give you a much better finish of the material you are sanding

Ken