I guess that was their way of adding replay value? I didn't even know those things existed.
I guess so. As I said before, I can be kind of a completionist when it comes to some collectibles in games, and I immediately got the first five red star rings in the first stage (since I already know where they are from the demo), and then I discovered that there are actually more things to collect even after you get all the red star rings. And that's why I'm still playing this game up to now.