RC car steering circuit: what does it do?
Since you said this was attached to a stepper motor, it's most likely a rotary position encoder.  The stepper motor can move the steering incrementally in small steps either direction, but the system still needs to know where it is, like how far from center.  That's probably what your geared thingy does.

If you hadn't said this was connected to a stepper motor, I would have guessed this is the stepper motor.