What does the relay behind the climate controls actually do?

I have an 88 p/u and that relay is missing. The truck has the A/C components, but it's all crapped out and I'm not restoring it at the moment. I am trying to get the heat hooked back up correctly. Right now it's running at high speed off an aftermarket switch installed by previous owner.

Anyway, do I need that relay for any heat functions?

 

I am not sure what the relay actually does but it is needed for it to work correctly. The wiring for the climate control separates easily. I would get one from a bone yard and put it back to way the factory had it.

 

Well, I know the heat is fully mechanical, IE the lever opens up the valve for coolant to flow into the heater core. I would guess that the relay energizes the compressor clutch for AC. I don't have AC so I'm not sure.