Good question. The primary purpose of a screwdown crown is to reduce the chance

of moisture getting into the watch from around the threaded crown. In the closed position the crown is tightened as close as possible to the case. The crown can't turn or perform its usual functions. In the open position it has to be first screwed away from the case. Once that is done the crown can then be pulled out as you would with a normal crown to perform its usual functions of setting hands and date, winding, etc.

John