I'm not familiar with the idle air valve on your specific vehicle, but on my older Mercedes, I leave the connector connected but pulled out a little, then can read the voltage. Some electrical things don't work unless they are connected (have a "load" on them).

Then Idle control valves in general allow the engine computer to make the engine run faster or slower. It may make the engine run faster to help warm up a cold engine, power the air conditioning when turned on, provide extra electricity if there is a large electrical load like headlights on, hot engine, etc.

And you would not want the engine idle to be high if the vehicle was in gear! (Should deactivate if in gear or foot on the accelerator.)