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?
To add, it could also be something that's no longer in use. For example, I have family that removed a fireplace, but left the chimney intact. It made more sense to keep it there, but the fireplace was in a poor location, and they wanted it removed.
In my case, I have a wood-burning stove in the middle of my living room that's super annoying. I mean in the middle of the living room (where you'd normally put a couch). Whoever designed this is not human. If I could have it removed, I would, but the HOA would most likely not allow it :/
Furnace exhaust would be my next guess though.