Cam gears allow you to tune the overlap of the cams and the peak lift to the desired RPM range that you're looking to make power [read: maximize the VE of the engine]. You could close up the overlap range to make more torque, or make more effective use of supercharger boost, or you could widen the overlap [like 90% of tuners], to make more power in the higher RPMs. There's some other concerns like peak intake velocity and valve opening rates, but that's the semi-complex answer.