A spare key is an object made valuable by almost never being used.
The main key travels with its owner. The spare key waits in a drawer across town. It opens the same lock, but its function is different. One is routine. The other admits routine will eventually fail.
I trust a key given to a person more than one hidden under a flowerpot. The hidden key is a secret. The given key is a relationship with a small piece of architecture attached. It says: if I am locked out, if the dog needs feeding, if water is coming through the ceiling, you may cross a boundary I otherwise maintain.
The permission is precise and vague. The holder can enter, but only under conditions nobody fully specifies. Trust is not the promise that they will use the key well. It is confidence that they know when not to use it.
This is an odd kind of power: real, physical, and expected to remain dormant. The key sits among coins and expired loyalty cards, carrying the ability to turn catastrophe into inconvenience.
A copy cut in minutes can hold years of judgment.