I have carried spare house/car keys for ~25 years. Because I learned to:

Once when I was 22 I locked my car keys on the front seat of the car, I could see them there. It didn't ruin the night, worried about it later. Luckily in the dark early morning there are a lot of certain types of people around. I chose a likely guy and he opened my car cleanly as fast as I could have with the key.

Once ~25 years ago I accidentally locked the door from the garage into the house. Had to go to a phone, call a locksmith...it was in the early evening so premium rates. I watched the guy make a key from scratch just by using their "special" blanks and jiggering it in the lock...probably took about 5 mins.

Anyway, my actual "contribution" was going to be: don't store your spare keys in any pouch/case/wallet/etc. that you also store ANY type of ID in. Oh sure, the guy will use the address/license etc. info to return your keys, sure... The corollary is that if you lose your house keys (with ID) you MUST re-key your locks ASAP. Even if somebody returns the keys. This may be obvious to people here, but I was surprised (when speaking to groups) just how "unobvious" it is to people.