Power is force multiplied by velocity. The engine power is actually (relatively) constant regardless of the gear. So when people say there is "more power" in a lower gear, it's the common misconception that "more powerful" is "more forceful" but that's only part of the equation.
So if P is constant, then that means if you can combine a large force and a low velocity or a low force and large velocity for the same power.
When you put it in a low gear, you produce a large torque -- or a large force -- and a low velocity.
For example, if you are towing a trailer or trying to climb a very steep grade, you need the force to be large which is why you put it in a low gear. If you are on something slippery like snow or ice, a high gear will keep the force at the wheels low so the tires don't exceed the coefficient of friction and spin.
