[2021] So... Rusted cars... How do I get rid of/use them?

Several things.

1. They look cool.
2. They tell a bit of an implicit story. Environmental storytelling in the BioShock sense, but much lower key.
3. They block space that you can free up later through effort. It's a form of progression. Similar to destructible rocks from StarCraft II. So the map changes over time.