Rms value and average value for a periodic wave
A resistive heating element gets hot when connected across 220V AC mains, even though the mean (average) voltage of the peak voltages (+311V and -311V) is 0 volts. Many loads don't care that the voltage reverses polarity with each half cycle. The 220VRMS value gives a more practical way to estimate the real power delivered to the load. For non-ohmic (inductive or capacitive) loads, there is a power factor correction, but as long as voltage and current are in phase, VRMS = IRMS * Resistance.