From memory it is there to block off one exhaust bank at idle and send the gasses across the inlet manifold to warm the engine up faster, If you open the throttle engine vacuum drops and the valve opens. I think it runs to an engine water thermal switch on the front of the manifold so once the engine reaches operating temperature it shuts off the vacuum to the switch. My guess is the person regoing it wouldn't know it wasn't there unless you make it obvious, so i'd get a new engine pipe made for that side. You'll need shorter exhaust manifold studs for that side.

It is called a "Valve and Actuator, switch - early fuel evaporation". Probably designed to get the inlet manifold hot fast so fuel doesn't condense.