Multiplayer, 4 with Wii U pro controllers playing and one with the gamepad in complete control of taking pictures and videos.
They would have complete control to angle the camera on the gamepad all throughout the stage without effecting what's on the tv. They can pause, rewind gameplay, take pictures and videos without effecting the tv.
They can edit those pictures and videos to save them to the Wii U and upload to Miiverse while the match is still going on.
There could also be more gimmicky things to do like the whole stage control concept, item control and a much easier stage builder tool. But a great camera experience during gameplay seems more realistic.
As far as single player I can't see it being used for much besides off tv play, map of the single player area or a quick menu.
They would have complete control to angle the camera on the gamepad all throughout the stage without effecting what's on the tv. They can pause, rewind gameplay, take pictures and videos without effecting the tv.
They can edit those pictures and videos to save them to the Wii U and upload to Miiverse while the match is still going on.
There could also be more gimmicky things to do like the whole stage control concept, item control and a much easier stage builder tool. But a great camera experience during gameplay seems more realistic.
As far as single player I can't see it being used for much besides off tv play, map of the single player area or a quick menu.