Same reason your parents don't put you on a TREK bike to teach you how to ride. You use a tricycle, then a fat-wheeled bike, then a TREK. But if you think it's fun to ride a tricycle later, you can still go back to the TREK w/o going through the fat-wheeled bike.
Okay let me see if I can explain this. Let's say that I am running at 3.2ghz stable and for whatever reason and go back to stock which would be 2.4ghz.
Now I decide to back to 3.2ghz. Is it okay to go straight from 2.4ghz to 3.2ghz without causing any damage?