You use it on a seperate suit inflation system like an argon set up... what it does is release the pressure ( vent to atmospher ) over pressure that may build up inside the 1st stage should the interstage pressure of the reg increase.. due to seat wear etc...
You don't need one on a standard reg set as the second stage vents the pressure... it freeflows if the IP creeps up.
Underwater Explorers
Check out the full argon sets... 1st stage.. overpressure valve and hose... If the IP creeps up there is know where for the extra pressure to go... maybe blow the suit valve and force fill a drysuit... obviously with serious results