Gearing in a model airplane has the same function as the gears transmission in your car. Shifting into low gear allows your car to climb a steep hill. The gearing increases thrust and decreases forward speed. In a model the same thing is true. The geared motor turns a bigger prop more slowly.
More thrust is delivered but a lower speed.
Putting a gear on a 25 motor makes the motor act like a four stroke 40. Gearing is usually a big advantage in Sailplanes because it increases the climb performance. In biplanes and in scale models of planes with radial engines the larger prop of the geared motor is a definite advantage. Over the years two basic rules of thumb have evolved. These rules involve gross weight and wing loading limitations.