don’t know about the rules in your state, but I’m pretty sure in WA if you dont have a licenced car to attach a number plate to then you have to hand it into the licencing centre for holding. I’d be interested to know if that is true not just here but anywhere??
In Queensland, other than the original purchase price (or the transfer fee if you buy an existing plate), you retain ownership of the plate, the right to display the plates on a Qld registered car, and the physical possession of the plates even if they aren’t attached to a car. No annual fees either.