Its because you are getting about twice as much power per pedal stroke. Riding platform pedals you cannot really get much power on the upstroke since you are not connected to the pedal. Now that you are connected to the pedal you can get that wasted power going uphill and you can definitely tell a difference. I ride a bike with platform pedals to work and I hate climbing any sort of hill or trying to accelerate. (Its a half mile ride, not worth clipless)