My experience with weights and clamps is thus:
1/ Coupling the record tightly to a mat often gives an over-damped and lifeless quality to the music. I rarely (pretty much never) like the effect even on those decks designed to have one (I know my deck uses one but then the mat arrangement is very different; as is the bearing).
2/ They help couple bearing noise to the record and thus to the stylus.
Audiophiles love accessories (I've got at least 3 different clamps).