Nothing to do with performance, all about longevity. The valve heads recede slightly during use which makes the valve stem sit closer to the camshaft. This means the cam-bucket clearance reduces with wear and (if you play the game long enough) you end up with a valve that is always slightly open. The worst issue here is that the valve then overheats and gets degraded and the seal is lost. I guess it does become a performance issue at that point... Valve clearances are set with the engine cold because operating heat causes the valve to lengthen which means the real clarance when running is really very small.
