They are basically used to get the advantage of more lift without using a big cam, like said above you would multiply the lift times the ratio of the rockers. They are an easy way of getting close to .300 lift with a stock cam. But, where cams have the advantage over rockers would be the profile of the cam....lift, duration, lobe separation. Ratio rockers do not increase duration much if at all, or change the lobe separation of the cam, they increase the lift at the valve and maybe the ramp. You have to be careful and check for coil bind when using ratio rockers, they can and will coil bind, especially when you have a big lift cam and not enough spring.