new question about motors

[hickston]

John really helped me out with skimming my motor, ie, it works again, but then ive had other advice about motors which is in conflict. and ppl dont need to skim motors...i look at the pits and everyone is skimming motors every race. ive 9 brushed motors now (and a couple of brushless)...can someone explain why the I need to skim every coupld of races?...yes i know they dont work if i dont, but why?

Ta
Stu