Well, UV filters cut down on the amount of blue light getting to the lens (you can get different strength filters) and they also can somewhat reduce the amount of haze you might get in an image.
As a protector they can work really well, and I guess these days with people shooting digital a UV filter isn't really necessary (colour film has an actual physical property that reacted to the use of UV filters).
But yeah, as the post above says, there's no need to stack them.
Mark