I could be wrong, but I think this feature is supposed to allow you to use your saves from your digital games on a cart. Still seems kind of pointless to me though, but it could be for getting the save from a game demo you try on to a cart if yo end up buying the actual game.
The feature is intended for people who have too many games to fit on the SD card. It allows you to delete the game (to create free memory) with out deleting your save data. Apparently before deleting the game erased your save as well.
This is more than the Wii U currently allows. No way to back up your saves still on Wii U.