The PWM is a technique which is used to drive the inertial loads since a very long time.The simple example of an inertial load is a motor. Apply the power to a motor for a very short period of time and then turn off the power: it can be observed that the motor is still running even after the power has been cut off from it. This is due to the inertia of the motor and the significance of this factor is that the continuous power is not required for that kind of devices to operate. A burst power can save the total power supplied to the load while achieving the same performance from the device as it runs on continuous power.
This article discusses the technique of generating a PWM wave corresponding to a modulating sine wave.