Well, someone at one time seemed to think that loss of air-conditioning in the flight deck was a grounding item. If the flight deck flow control valve doesn't open normally, it can be pulled open. If it can't be shutoff normally, it can be shutoff manually. However, mid 70's, someone thought that it was not necessary and they took that part of the system out. I have never seen it intentionally used, but it has been unintentionally shut off many times. Interestingly, the temp control boxes will still drive the temperature system to cool with the master switch off if the system keeps running and there is an overheat in the ducting. I can only guess that the designers thought that someone would manually turn the system on?