Have you ever locked yourself out of your own house?
Although it's rare, getting locked out of your own home can be a real pain in the neck, especially if the weather is bad and you're stuck sitting outside your own house waiting for somebody else to unlock the door.
The solution is pretty simple. Give a spare key to a trusted neighbor or hide an extra key somewhere outside the house. This is not only useful for those times when you lock yourself out, it's also useful for occasions where you can't get home, and you need to let somebody else in to feed your pets or walk your dog.