They allow the board to stay with the rider while airborne but they are also easy to remove in the case of any "board-off" tricks. Many also have a grab handle in the centre of the board in order to facilitate the removing of the board during a trick.
These enable the board to remain attached to the rider when airborne, though they can be quickly detached for "board-off" maneuvers. Several models include a central grab handle to assist in disengaging the board during tricks.