It's basically caused by the hammering of the valve and valve seat as they open and close many times per second. Over time, this hammering wears them down and reduces the gap between the valve stem and the tappet. It's not much wear (Hundredths of a millimeter change) but it does make a difference.

If you are doing a valve adjustment yourself for the first time, remember to make sure if you are adjusting an intake or exhaust valve and double/triple check your adjustments before you put the valve cover back on.