The switch powers the ESC on/off. If off, it won't arm. This means less spark on hook-up since it doesn't have to charge the caps AND beep the motor. Also used for ESC/radio calibration and basic programming. Yeah, that's not a big deal, but it's easier to switch radios (if someone else is bashing with me and need to use my extra DX3.0) and do the binding process by powering up with the switch instead of trying to hold the bind button and connect the batteries. Not a big deal, but it's a reason.
