A power capacitor filters out all of the ripple on the input voltage. This ripple is due to the high frequency operation of the speed control. If you don't use the power capacitor on high-frequency ESCs you run the risk of causing internal damage to your speed control. Using the capacitor will result in an approximately 10 percent increase in the RPM of your motor at partial throttle. It will also decrease the operating temperature of the ESC by 10-15 degrees, making the ESC more efficient.