Keep some on hand

If you are travelling or going to go through a period of time when you won't be able to afford or get to a mechanic, or just want to be prepared-- having a small supply of vehicle fuses and relays "just in case" is just a good idea.  The same reason you keep a jar of used-but-good nails and screws in the garage in case you need one it's there, and done.  Being prepared can make repairs easy and fast, and cheap.  These are ready for customization and use-- no good reason not to have a few in your car and your toolbox.