They are gritty, abrasive substances that come off tires as they wear. Also, chemicals from diesel smoke and other emissions in the atmosphere settle on your car's surface. If this residue isn't removed immediately, permanent damage to the finish can result. Insect residue and bird droppings form acids that immediately start to eat away a car's finish. Rust is simply the oxidation of untreated metal surfaces when they contact the elements.
Moisture is the main cause of rust.
Since dirt attracts and traps moisture, a dirty car is the instigator of almost all car rust, especially in those hidden areas behind the chrome and trim. They deteriorate the whitewall surface. A self-service car wash in most cases provides whitewall cleaner and the proper wash pressure to clean the tires. Or, you can go to a full-service car wash where they will clean the whitewalls for you. Lately, my windshield smears when I run my wipers.