Water Valve question

[gman]

This probably sounds ridiculous, but what does the "water valve" do? What is its purpose? I see it has a vacuum fitting on it but mine is not hooked up. The vacuum or the hoses. The lower heater hose goes directly to the intake. After reading a recent post by 6772king, GMC Paul explains his vacuum connections. His diagram shows the valve "open with vacuum applied". My 72 manual shows "closed with vacuum applied". I'm uncertain what this is doing and its purpose. Maybe this is part of my problem where my temp gauge on dash barely moves out of the cold range. Maybe also the thermostat is bad but one of the housing bolt heads is broke and I haven't got a bolt extractor yet. Just recently purchased my 72. Any explanation as to what the valve does and how it works is appreciated. Thanks!