The cheap ones with what looks like an eyelash stuck on the playing area are perfect for bashing the delicate lens assembly mercilessly if that's what you're after.
The only proper way to completely clean an optical lens is very very carefully with alcohol and a special "q-tip" stick, if you are not up for the job then a good repair job could help (are there any left in the UK?).
The lens in optical players pick up dirt and if there are smokers in the room, the lens does tend to get eventually get coated in a film of gunk.
The operation only takes a few seconds and the results can be quite dramatic, as a clean laser lens means less digital error correction due to poor reading.
I strip my stuff apart every six months for a good vacuum and clean but that's probably excessive in most cases.