3D World is really a 3D Mario game but with "training wheels". At the beginning it plays a lot like a 2D Mario, where all you really need to do is get to the flagpole at the end. It's also multiplayer like the Wii 2D Mario. But you can only get so far into the game just getting to the flagpole. If you want to unlock the last several worlds then you need to go back and collect stars. If you want to unlock Rosalina, then you need to collect enough stars. It trains "flagpole Mario" fans to play "starfinder Mario" instead. Also there's one more little detail, but I think this is important. The game is actually in 3D.
Nintendo doesn't want to put serious effort into making a 2D Mario game anymore, so they tried their hardest to train their 2D Mario fans to like 3D Mario instead. Super Mario 3D World is meant to be a bridge game to train 2D Mario fans to like 3D Mario.