Control. If you ride/work in steep/rough enough country, at some point you'll be using your grip on the handle to keep your seat. That little thumb lever keeps you from giving it more umph when all you're trying to do is hang on...that unintended umph might be the difference between being on the trail and a couple 100 feet down the side of the mountain.
This ^^^^^ from my own personal prospective as well.A grip throttle would = an elevated cast most assuredly.
