Top comes out on the roof:
Not sure what happens below. Haven't gone under the house to inspect. Seems like a vent that travels through the wall space, but unclear why it would do that.
Any ideas what its purpose is?
It probably is a left over vent, but I'm guessing it drops right into the wall space, and down under the house. My plan, if its not asbestos, is to reuse it for a bathroom vent, which would solve both issues.