If, for some reason, you want to to take all your data from one Wii U and put it on another Wii U, you can now do that. I guess?
Wii U to Wii U data transfers are available as of the version 5.1.0 system software, which is rolling out now. The transfer ports over users, Nintendo Network IDs, save data and downloaded content including games.
The source Wii U will be formatted, so it’s not ideal if you have multiple consoles around your house (or in your holiday homes, you bourgeoisie). But if you’re upgrading from a Wii U Basic to Deluxe – perhaps because you ran out of room on the smaller HDD – it could save you re-downloading all your games and losing your saves.
The update also adds a number of other useful features and such, including the ability for system updates to install when downloaded during standby. Check the full patch notes for details.