I initially set/release the throttle stop to allow full throttle control with the twist grip (full slow). I then set the carb idle mixture, and throttle/carb link and sync. Properly dialing in the idle mixture (carb) yields the lowest speed at which the motor runs smoothly, warmed up and in gear (with back pressure). As a final step, I adjust the throttle idle stop to match that rpm setting. Having done so, clockwise twist grip travel will be limited by the idle stop, like F_R says, and inadvertant stalling will be prevented.
Idle speed should typically be around 650 rpms, although I usually do it by sound. With a well-tuned motor in excellent condition, though, a lower idle speed could work fine. And with a tired motor, the idle speed might be higher -- though should never be so high as to risk gear damage or to threaten safety. If a motor has to be slammed into gear to keep from stalling, it needs maintenance or repair.
I don't know if the throttle stop is useful for other purposes (setting a trolling speed, for example). I just never use it for that purpose.