Why is the motor to be equipped with an encoder?

A student from a college visited the factory and happened to see the workers installing encoders for a batch of motors at the assembly plant. A few curious little brothers asked the technicians who instructed the workers to operate: Why should the motor be equipped with an encoder? The technician explained that the encoder is a signal sensor that transmits the running state of the motor to the motor controller in real time to realize the speed regulation and start and stop of the motor.