I'm going to put my two cents in by saying that a prop creates three things known generically as p factor. They are differential blade thrust, slip stream over the tail and gyroscopic precession. These three forces act on the plane depending on your flight configuration Without going into a long winded discussion, the right thrust helps compensate for these factors and generally you will have an easier time flying your plane. Its like putting washout in your wings; Don't have to have it, but it makes the plane easier to fly. If you want to know how a plane fly's, read "Stick and Rudder", an old book but probably one of the best on the subject.