What is a clutch brake used for on a tractor trailer truck?

When you depress the clutch pedal all the way down the clutch brake stops the input shaft from rotating. This allows the transmission to be put in gear. Never depress the clutch fully ( all the way to the floor) when the truck is moving in any gear. The clutch brake is about the size of a compact disc and cannot stop the input shaft when the truck is moving in gear. This will destroy the clutch brake.