The heater control valve is operated by the slide on the dashboard. when you move it over to hot it opens the flap up to the heater core and from there it opens a valve that pulls vacuum that sends the vacuum signal to the control valve so the valve opens and hot coolant flows into the heater core.
Many remove these, as they find them troublesome, what happens is in the summer when it`s not in use, the coolant in the heater core doesn`t flow, so in time it will set rot in one place of the core, so the core don`t last as long, and some heater cores are down right a pain in the assets to change. I dunno what they were smoking at the factory when they came up with the design where you gotta rip the whole dash out just to change it over.
The next issue is, these valves mostly came on vehicles with A/C. Why was because where the Cold and Hot flap is, the flap is a very thin piece of tin, and if the heater control valve is removed, the heater core stays hot, then in turn it makes the flapper get hot. So when the A/C is in use, it don`t get as cold as it should because it`s drawing the heat off the flapper door. So most reinstall the control valve. This is what was happening in my 86 cutlass, I didn`t want to reinstall the valve because it looked bulky and got in the way.
So I removed the cover to the flapper. Then I went to a neighborhood were fresh houses were being built, looked in the dumpster and found some left over pieces of that aluminum sheet coated house insulation. I cut out 2 pieces that would fit on both sides of the flapper door, then I used a layer of silicon to tack it down. The end result was it worked like a champ.