Car detailing involves cleaning the tires, interior, and undercarriage, as well as polishing and waxing the exterior. It might not seem like there are reasons to get your car detailed besides aesthetic ones, but detailing is actually an important part of preventative maintenance.
Your car's paint acts as a protective layer between the metal underneath and outside elements. Keeping your paint job lustrous with a wash and wax serves the very important purpose of preventing rust, corrosion, and chipping, which can damage other parts of your car and just don't look good. Plus, dirt in or on your headlights can significantly reduce visibility and available light.
The same goes for the undercarriage. This is where the majority of road salt and other harmful materials build up, so washing it regularly will prevent rusting. Rust is a serious problem-you have to remove it as soon as it starts because it spreads so quickly. Prevention is the best medicine.