It is very necessary to restore and balance out your body's own hormone production in order to maintain gains and stop from crashing. If you don't do this, your body won't readjust until you've crashed. Think of it this way: While on gear, your on the high end of the spectrum. If you don't use PCT, your body will drop to the negative end of the spectrum which will cancel out everything you've gained while on. This can have adverse effects on gains, sex drive, mood, and energy levels to name a few.