People use it because that’s where the sellers are at. I also liked Craigslist before but Facebook ate their lunch plus one slight advantage from marketplace is seeing who you’re buying from beforehand. It makes it a lot easier to weed out fake listings when you see someone just created their account this year or if they have bad ratings.
Vehicle listings are absolute garbage though as the filtering options are super basic or ridiculous like you can filter by the color of a car but not engine size. I don’t know of a single person who searches for cars/trucks based on their color.