a couple hundred dollars in scrap, thanks platinum

They're a motor vehicle part that improves a car's emissions. They're on the underside of the car, as they are a part of its exhaust system. As other commenters have explained, they contain a lot of rare and expensive metals like platinum and palladium, making them quite valuable. As a result, there are many cases where people will go under parked cars and cut them off while the driver is away, then sell them to less scrupulous buyers for a quick buck. This, of course, fucks up the car, hence the post.

It's the part of the exhaust that lowers volume and I think emissions too

When a motor vehicle burns fuel, the exhaust is routed out to the back of the vehicle through the exhaust pipe until it reaches this part, the catalytic converter, and then the muzzle. The catalytic converter has surfaces coated in platinum or other rare heavy metals that act as catalysts in a process that breaks down the most dangerous pollutants in the exhaust very quickly, reducing pollution.

The metals used as catalysts are very, very rare (some of the rarest metals on earth, even) so they fetch a very good price.