HJL,
Almost all vehicles have a purely mechanical lock in the door for when the battery is dead or similar failure, and these are the simple flat keys which you can just keep with you in your wallet or keychain or hidden somewhere instead of the fat remotes and can be duplicated most places that do other keys. If you get a spare security key, you can keep it in the (locked) glove compartment or some other secure place; you’ll need the anti-theft feature of the fat key to start the car. There is also the common scenario where you lock your key in your car. – T.Z.