The beacon comes on to alert those on the ground that the a/c is about to move or an engine is about to be started. Tower/ground clearance depends on the airport itself-most places we operate from we do not contact atc until taxi. Needless to say the cockpit should be in contact with ground staff throughout.
In the past we would sometimes use the beacon to alert the ground crew that we are ready and they would put on the headset and contact us.