What is the function of automobile idle motor?

It is used to adjust the idle speed according to the engine load at idle speed.

Idle stepper motor is installed on the throttle body and used by the engine controller to control the idle speed of the engine. When the engine speed deviates from the idle speed, it is adjusted by the throttle valve to control the amount of air entering the intake manifold, which is mechanically operated by the accelerator pedal wire rope.

The throttle body has a bypass air passage to provide air when the engine is idling (when the throttle is closed). ?

When idling, the cross-sectional area of idle air passage is changed according to the temperature and load of the engine, so that the engine has the best idle speed under different working conditions.