1) I drive everywhere! I spend up to 4 hours a day in my car (twice a week while oldest daughter is in college) When I'm working it's an hour round trip. Grocery store runs, shopping trips, dropping teenagers off at work etc.....all this time in the car adds up, why not spend it in a clean space? We make our beds every morning so our bedrooms are neat for the day, why shouldn't the car be a clean/neat space too?
2) Clean spaces make me happy. (that one's simple)
3) It teaches the kids to respect my stuff. They spill something they have to clean it up. If I kept it filled with garbage they would treat it like garbage.
4) Protect my investment #1. Pens laying on the seats can poke holes, or leak. Half eaten drive-thru food stinks up the interior and leaves stains. ChapStick melts out of the tube etc etc. It's easier to just grab the stuff you brought with you when you leave the car then to scrub a seat for an hour trying to remove a stain.
5) Protect my investment #2. When I'm cleaning the car I'm looking at every piece of it. I see if a seat is tearing, something is broke or needs replaced then I can handle it right away. Small repairs are often cheaper to fix and prevent the problem from getting bigger.
6) I get an hour of me time! Nothing to do but daydream while I wipe and vacuum. (or the kids can help by taking their stuff into their rooms and wiping up with a damp rag)
7) It's super cheap to do it yourself! You could easily pay over $100 to have it done commercially, or do it for yourself for less then $3! Hubby detailed cars as a teen and swears by 3 things: window cleaner for windows, a Dawn & water combo for cleaning dirt and spills and Endust for the dash/doors....plus the vacuum cleaner of course.
8) Once it's done.....it's done! All you have to do is take the fresh garbage out with you each time you exit the car and wipe up any fresh spills. Easy to keep clean and admit it....it just feels nice driving a clean car. Should stay clean for a few months at least.
9) It might be your first impression! Ever offer somebody a ride? Would you really want a co-worker thinking your car is a pigsty? Or your kids teammate needing a ride home and you have to push garbage off the seat to make room? Embarrassing!
These days most of us "live in our cars" but they don't have to look like it! I drive through those 'brush free' car washes all the time, but as much as I like the car to look good outside....when it looks good inside I feel better too!