>>This doesn't seem to make a lot of sense.
>>The number is on the key.

Many new car keys come with a "key code". It is usually a tag that looks like a dog license tag. Many dealers will put it on the keychain with your new keys. I have also seen it stamped or placed into the owners manual. If you have the "key code", the dealer can cut you a new key on the spot.

Obviously, many folks lose the key code long before they lose the keys