What are the clouds for in Super Mario 3?

Answer 1

The clouds in the backround are just for decoration. In world 5 in specific levels, they can be used to stand on. They are very useful in the game. As an item, Lakitu's Cloud (or Jugem's Cloud, as it was originally known) can be used to bypass any level, except the Mini Fortress & the Airship Boss. The catch is, that Mario or Luigi must complete the level following the skipped level, because if you lose a life, you will be automatically taken back to the last level that you completed.

Answer 2

there isnt any Yes there is...

if you mean the Worlds in that game that each Level is on...

the world with clouds is after the pipe world

it's got to be at least World 5. if you want to know, just play the game.