Like Taurine, I keep my wallet in my car too. Everywhere I'd need my wallet for, I'd need my car for. If someone steals my car, then I'm not really that concerned about the 20$ in my wallet...
But you have to have the keys, get in out of cars, houses, locks. You can't do anything without keys.
And of course the phone, because I need to call the cops when my car/wallet has been stolen. Also handy for taking pictures.
And CDs. Although I only live 3 and 5 minutes from my work and school respectively, every time I go for a drive I seem to put another CD in the car.