You can see in this video that the water in the car has caused rust. Rust causes more rust, so having rusty water sloshing around in your car is not good.
The moisture in the car can also get into electrical systems, either directly if they are submerged, by capillary action as water is pulled up the wiring loom, or just by condensation.
Dirty rain water getting into the car will also cause mould and odours, which require treatment if they are not to become a health hazard.
Share this video