What are the Star Coins for in New Super Mario Bros. Wii?

Collecting all three star coins on every level in a particular world opens up a corresponding secret level in the correspondingly secret World 9. For example, if you get them all in world 3, you unlock world 9-3

You need to collect every star coin from all nine worlds to achieve the second and third of the five save file stars, which indicate how much of the game you have completed.

You can also use them to buy videos like "super skills" Or "Star coin".

You could also use them to buy hint movies in peach's castle (at the complete left of world 1).

They show certain secrets, such as the location of a secret goal or how to gain infinite 1-ups.

The existence of Star Coins serves for many useful purposes, such as buying hints and movies in the castle, getting more points, etc.