[Jake.]

If you get all 7 you can turn into super sonic/tails/knuckles if you’ve got 50+ rings, which makes beating bosses a doddle (that and you need them to unlock the true final boss/ending if you’re playing as sonic).

The bonus game at the checkpoints are called blue sphere. To get unlockables in the game, you have to collect all blue sphere’s and rings in each one. Some rings will be already visible in the stage, others will be hidden. These rings can be revealed by surrounding blocks of blue spheres completely by an outline of red spheres—the entire block will then be turned into rings. So lets say you come across a large square consisting of, say 24 blue spheres, collect the outer blue spheres and all of the others will turn into rings.