1A Skylight Filter

I recently purchased a 10" CAT that has a 1A skylight filter which treads onto the rear cell betwwen the telescope and the eyepiece. I don't know anyone who uses one. Is the only function to keep dust and dirt out of the telescope? Does the extra piece of glass in the optical path effect the overall figure of the telescope? What do people think, to use it or not to use it? I live in a light polluted sight which has average to poor seeing conditions and wondering if I could tell the difference anyways.
Thanks,
-Brian White