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?
Most vents share that flaw but most people have no idea that they do. There’s no way they can prevent some blowing rain into them. Most of the time it doesn’t matter but if the vent is abandoned into the attic, what little bit of rain does get through it will dump it straight into the attic ceiling. I’ve seen this way too many times.