Journey air leak in air horn system

[RKL2]

Checking out Journey before trip south and when I shut the engine off I heard a loud air hiss..Finally found it at the canister which supplies air to the air horn solonoid,, It was leaking from the air shut off turn handle just before the air guage...Tried to stop it but finally removed the fitting that contained the air stop valve and handle and attached the incoming pressurized air hose directly to the valve that goes into the canister and the guage....no leaks now but I wonder why the valve assembly has an air shut off inline except to shut the air off if you had a leak in the air control solonoid or the lines to the horn...Any other ideas as why this shut off is there? Also it has a shraeder valve as part of the assembly...What is this for? thanks RKL