Many old vehicles have registration numbers attached to them that can be of value and possibly sold on to get some extra income from. Rusty old cars may be on the verge of dying and possibly scrapped but saving their number plates could mean you're still not out of pocket.
Old vehicles tend to have short combinations and many prefer old style combinations of number plates. In many countries the laws of transferring a number plate are different. In most countries you simply will have to give up the number plates regardless of what they mean to you or someone else. In the UK, it is possible to retain a number, however the car must be roadworthy when the application to retain is made.
So if you own a classic car or a old rust bag that's on its way of leaving the motoring world, keep it healthy especially if it has number plates that may be of value, and when possible and early as you can, save the number plates as they may become custom number plates to others.