Roller weights are used for fine tuning your scooter to get the most out of what you need from your scooter.
Light rollers are good for quick acceleration, stop and go riding and hills, but you loose top end speed. Heavy rollers will give you a higher top end speed (long roads) but you will loose low end acceleration.
If you?re a heavy person (and/or) if you have a stock motor you will want to lean towards the lighter rollers. If you are a light person (under 140lbs) or you have a big bore you will want heavier rollers