It's called a "heat riser", and it's intended to warm up the intake by routing exhaust gas under the intake during cold starting. If it wasn't necessary, why did GM go to the trouble and expense to install it? I'd suggest getting it working freely, and replacing the spring in the process if you can find one. The spring is supposed to hold the valve closed when the engine is cold, and the weight opens the valve as the manifold heats up.
Jerry

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