The main reason for using a reducer is to decelerate. As the name implies, the reducer is used for deceleration. The commonly used power source is a motor. Generally, the speed of a four-pole motor is 1400 rpm under 50HZ. (The frequency will affect the motor speed.)
But your working machine does not need to be so fast. There are still limitation to use frequency converters. Whether the frequency becomes lower or higher, it will cause voltage to change, and the motor output power becomes lower accordingly, then the motor cannot work long time. It can be simply burn out.
Secondly, according to the formula P=(V*M)/(9550*η), we know that the speed and torque are inversely proportional when the power is rated. The lower speed, the greater torque. That is to say, you can use a smaller prime mover to drive a larger machine.
At the same time, the reducer can also meet your commutation demand. Many machinery and equipment have special installation due to space, plan and other reasons. A suitable reducer can meet all your needs under this circumstance.
With having these function, reducers are widely used in so many industries, from digital tools, mechanical arms, to communication equipment, production automation etc.