>>1560636Input power to pedals -> output power to turn wheel
Gearing changes how much movement you get from the back wheel for the same amount of power you put through the pedals.
So if you have conditions where it's difficult to ride - such as up hill, or over rough terrain - you want more power concentrated on moving across a smaller distance, to move at all. When you need to go fast, you want to be using that power to move as much distance as possible.
