Why a spare is about having a backup
A spare is a second working key kept in case the first is lost, damaged or with another driver. The point is not a copy of the blade but a complete key that opens the doors, starts the vehicle and, where your key has buttons, works the functions your vehicle supports. Getting one while a working key exists usually avoids the more involved all-keys-lost process later.
What goes into a working spare
Depending on the vehicle, a spare can involve cutting a blade, registering a transponder or smart key to the immobilizer and pairing remote buttons. I test the spare in the doors, confirm the vehicle starts with it, check your existing key still works, and test the buttons your vehicle is equipped for.
Planning spares for a household or business
Many vehicles accept several keys, though the limit varies. Households with more than one driver often keep a key per driver plus one stored at home, never inside the vehicle. For company vehicles I need authorization from the business as well as ID.