Actually it is used to RAM outside air into the cabin (use below 1 PSI please) and is used in the smoke drill to clear smoke from an unpressurized cabin.
In the event of pack failures etc it can provide outside air to ventilate the cabin.
Pressing the valve with the packs in Automatic will also cause the outflow valves to open rapidly and depressurize the aircraft. Hence the caution about 1 PSI.
Cheers
Wino