01. To protect it from the elements, contaminates, and oxidation.
02. Simply Cosmetic, having your vehicle looking great inside and out.
03. To increase or maintain it's value. 
04. Health and comfort.
05. Safety. 

Wax, sealant, or ceramic coating will add protection to the paint work. This will also extent the life of the paint and keep it looking clean and shiny. Keeping the interior clean, by steam, will eliminate dirt, bacteria, dust mites and other allergens. A windshield treatment  will reduce
night time glare, repels water, and increase visibility. Great for rain or snow. The list goes on, but I just wanted to keep it short and simple.