Investigating further I found this post from 2003:
"The primary purpose is to let an engine rev higher without valve train problems. In a standard OHV configuration, each valve spring must try to control the weight and inertia of the lifter, pushrod, rocker arm, and valve. As RPM's increase, the intertia gets harder to control. One solution is to install stiffer valve springs, but that has its own set of problems. A rev kit adds a second set of springs, which control the weight and inertia of the lifter, removing a large amount of work from the valve springs (and the pushrods, since they no longer have to work so hard to keep the lifter riding on the cam). Since the lifter is one of the heaviest (if not the heaviest) part of the valvetrain, then controlling it with its own spring greatly reduces harmonic vibrations in the valvetrain at high RPM's and lets the engine rev higher."
