To break in the springs and such. It just allows your gun to shoot more consistantly before you go messing with it.
This is if your talking about a new gun.
If he told you to put 5 cases through even if it isn't new before you mess with dwell, well, he's an idiot.
A cheaper alternative is to just put a few tanks of air through it and dry fire.
If you hear before you see me
I got king kong in the trunk