Motor question - Prime/Idle speed
 

This is a newbie question.. hope you don't mind answering. I have a Mercury
15 HP 2stroke manual start motor. There is a pull-able and turn-able thing
called "Prime / Idle Speed".

1. What is the Prime/Idle speed thing for?
2. Is it same as the "choke" for stopping the engine?
3. After staring the engine, I never touch it and always put it as
clockwise (fastest I guess).. why do some people tell me to adjust the Idle
speed by doing anti-clockwise? why need to adjust idle speed?

Thanks...