The "use" should be fairly obvious. Senior figures at Nintendo (notably Iwata) have commented on numerous occasions that they think games controllers are intimidating for non-gamers, and that they have too many buttons, sticks and input devices. So, they want a controller that has as few buttons as possible to be more approachable to people who haven't played games consoles before. On the other hand, they also want a full suite of buttons to accommodate whatever third party games they may get (however few).
This design accommodates both seemingly contradictory requirements. When a non-gamer picks up the controller, they only see a big screen with two control sticks, no more intimidating than their phone or tablet. When they start the game, only the buttons that the game actually requires appear. And, instead of being labelled with arbitrary letters or symbols, they're labelled with what they actually do, which makes it a lot easier for new gamers to realise which button does what. So when you start the tutorial for Super Mario Multiverse, there aren't any buttons on screen, just the stick for you to get used to moving Mario around. Then a jump button appears, and you learn to jump. Then a fireball button, etc., etc.
At the same time, the design would accommodate an arbitrary number of buttons to accommodate the control schemes that existing gamers are used to (and potentially even add functionality like allowing players to adjust the position and size of each of the buttons).
Of course, the whole thing would depend entirely on good quality haptic feedback, and I have no idea how likely that would be. Personally I'm still of the opinion that this is fake, but if Nintendo does go this route (and given the patents they've filed, they've certainly been considering it), then it seems clear to me that they're doing it for the reason above.