suppose the counterbalance the motor. I believe every unit comes with one installed. 5's for the 7424 and 6's for the 7336. I bought the bigger one per PC's directions and figured why not since its so darn cheap. Others say they've run the 5 without issue for many years with bigger pads and bigger backing plates.
Porter Cable makes an optional counter weight available for their 7424 polisher if you upgrade from their 5-inch backing plate/pad to a 6 inch you can substitute the optional counter weight for the one that came on your machine.