Like Aldnorab, I've used them to raise tubes for fit. I noticed that they fit so tight that it is hard to leave them installed and pull the tube out of them. Struggling to get the tube out of the tube saver puts stress on the socket.
If somone designs a quality socket saver that fits snugly into the socket and tubes can be removed more easily, that would make sense.
Here is where a socket saver is useful: If you roll old tubes a lot and don't like to spend 10 minutes cleaning the corrosion off the pins with steel wool. The corrosion on the pins is abrasive and will wear out your socket and cause poor connections. your socket ends up cleaning the pins.
Jerry