The choke coil on your carb which is the source of the end you know about has a small amount of vaccum on it so it pulls the air through that tube which is in the heat crossover passage on your manifold. It picks up heat and then heats the choke coil which operates your choke. I think (can't remember for sure) the other end should go to a nipple under the lower edge of the air cleaner housing but don't quote me on that. It might just go to a nipple above the choke plate on the carb which is really the same thing. It gets filtered air that way. I guess they don't want some bug plugging it up
If you convert to an electronic choke or a different carburetor than the Rochester you might not even need that and you can plug both tubes or remove them altogether...