Usually it's for one of two different temperature controlled ported vacuum switches for EGR control. The second one would be in the top tank of the radiator. CCEGR or somesuch nonsense, was part of the whole Lean Burn thing with a feedback carb (Quadrajet, I believe) in the late 70s. Plug it or use it for the oversized temp senders that come with aftermarket gauges.
The pic in post 4 is the '88-'92 2 bbl. TBI manifold, that sensor is coolant temp input to the computer.