What is the black plastic key for in my BMW 323i for?

It's an emergency "wallet" key. Basically, BMW made it really small and lightweight, so if you lose your original keys (or lock them inside your car) you can still open it if you've stashed the "wallet" key in your purse or wallet. f.y.i. only use it in a EMERGENCY. i used it when i run marathons and it cracked in the igintion and i cound not start the car. BE CAREFULL