"Pushing" a high gear at a slow cadence is like power lifting. It is good for building up muscle mass and bulking up your legs, but it does little for your heart or lungs, and you can hurt yourself if you overdo it.
"Spinning" a lower gear at a rapid cadence is more like swimming. The rapid motion, with many repetitions makes the legs supple and flexible, it is highly aerobic, and the light pressure that goes with this style reduces wear and tear on the joints. With practice "spinning" becomes easier and more comfortable.