Dual overhead cams allows the engine to obtain higher RPMs and thus higher horsepower compared to an overhead valve engine with the same displacement and number of cylinders. Overhead valve (pushrod) engines are limited because the the pushrods will break if revved too high. A dual overhead cam engine allows for more valves as opposed to a single overhead cam engine and thus also produces more power. Having more valves allows an engine to "breath" better. There is usually one cam for intake valves, and the other cam for exhaust valves.