You go through a door, you see a stage it may be in two colors.
A few geometric shapes. It appears they had left over assets and decided, in some cases, sadistically, decided to add gratuitous stages, that you still need to collect them moons.
Most of these situations would be interesting if they took care to make something beautiful like New SMB Wii U that had a Starry Night theme, for instance.
It becomes even more trite when you can get 200 coins and buy the 2 moons post story-campaign-game in the store.
I’m sure there is something else, I really should take notes.