The screw keeps the throttle plate from wedging itself in the bore. Open it too much and the IAC will get confused. It needs to control all the air with the throttle shut. In an emergency like the IAC breaks I would open it up so the car will idle. Mark the position with a scribe mark first. I had mechanical idle controls on my old VW suddenly break and I had to do this to drive the car home.
I've seen some TPS sensors with with a little play in them so you could tweek them a little.
