The purpose is to balance out the Pitman arm as it directs the steering linkage through its travel. Ideally, the Idler and Pitman arms should be the same length.
The tendency to return the steering wheel to center is a matter of proper Caster alignment. The more Caster, the greater the resistance to turning the wheel and the more stable the steering is.
I alway thought it was steering axis inclination that returned the steering to centre on a bodies. Most a bodies normally have almost no caster by design in there alignment setup.
