They don't..........unless they are lighter than what they are replacing. Use of them is usually dictated by things like cam choice, milling, etc., with no thought as to the weight. Pushrods are part of the weight of the valvetrain, lightening them will allow for less weight that the valve springs need to overcome to return the valve to its seat. Lifters, pushrods, a portion of the rocker arms, top collar of the valve spring are parts of the moving valvetrain. Lightening as many as possible will yield an engine that will spin-up quicker.
....may allow the engine builder of your choice some flexibility on installation. They don't do much to increase performance by themselves, but they may help someone put together an engine with other performance parts.
The adjustable pushrods don't really do anything for performance. They are good for changing your cams without taking the rocker covers and rocker arms off. It also makes it easy to change your cam chain tensioners without taking the covers and rockers off.-2$en#e-