You kinda need one for a MAC or such things that have fast threads and don't have a ratchet or other anti rotation system. Otherwise your can can unscrew while shooting. The silicone sleeve lets you hold a little torque on the can while shooting to keep it from rotating off.
I take a break when the gun and can start getting hot and shoot something else while it all cools down. This saves the can as well as the barrel. Bolt open and muzzle up lets things cool down a lot faster - the chimney effect.