When the engine is cold, the valve is closed and restricts exhaust flow. That restriction forces exhaust gasses to pass through the intake manifolds exhaust crossover port over to the other side. This is done to help bring the engine up to opperating temperature faster.
There is a metal spring that loses tension when it is heated up. That allows the valve to open when the engine is up to temp. If you have the exhaust crossover port blocked off, there is no reason for the heat riser. Most original heat risers aren't in the best condition after all these years and no longer work properly. You can buy a kit to rebuild them but most people just weld the flapper in the open possition and call it done. If you choose to do that, the car will take longer to get up to operating tempurature and may run rough on startup.
