Its needed because when your foot is not on the gas the throttle plate is completely closed and shuts off air to the engine. If the engine has no air it obviously cannot idle. The "Stepper motor" controls a small pintle that fits into a passage, this passage channels air around the throttle plate. When the motor is retracted the engine idles up, when it extends it idles down. The motor also compensates for engine loads at idle speeds. You turn the A/C on and it drags the motor down, the "Stepper motor" brings the RPM back in range. The same happens when any load hits the engine at idle.

With that said, it should be pretty obvious what the symptoms of a bad or dirty motor are. Your engine will not want to idle. Or it will idle low and stall when a load such as A/C or Power steering.

To service you will have to remove the motor from the plenum and clean the end of the valve. You should also clean the passage that it fits into with carb cleaner. If the symptoms remain after cleaning the motor more then likely needs to be replaced.