For adjusting to how heavy your gate is. A light gate needs less torque at the motor to move it than a heavy gate. The circuit senses this torque by measuring the electric current going into the motor.

"So why not just set it to maximum?" you ask. Because this setting is also to sense if the gate stops moving, for let's say, there is a car in the way. You don't want it too high until the gate doesn't stop moving even if it hits something in the way.