Here is a pic of a vent installed by the guy who put down the plastic crawlspace flooring and insulated the cinder block walls.
He closed off all foundation vents too---with the styrofoam insulation.
When heat OR a/c blows, this flap opens and heated/cooled air is blown into crawlspace.....it's like a 6" section of dryer duct.
I'd never seen this and gotta know if its goofy or not.

Depends on whether someone's living in the crawl space. That's the only reason to seal/insulate/condition the air. Or, something is being stored there that needs "indoor" conditions.
But, by far the most likely reason is that an idiot installed it. Idiots are everywhere.

Only thing living down there are some mice.
Not enough room to store anything,,just enough to crawl around comfortably.

It's an air inducer that allows a limited amount of conditioned air into the crawlspace. It's just a much cheaper alternative to putting in a dehumidifier down there.

Possible Radon Gas abatement?

Yep

Just to be clear, I didn't say it was necessarily a GOOD alternative.....haha

I used to install radon mitigation systems. They were always negative pressure, i.e. the crawl space was subjected to a low vacuum.

Lately, my brain has been a radon mitigation system. Negative pressure and a very low vacuum!