Скачать или смотреть Why the insta-shield is awesome.

The insta-shield gives you a few invincibility frames - making it a very helpful move against bosses and enemies, and widens your hit-box radius.
We all know this.
But~

In the Classic games, if Sonic jumps while he was rolling, there would be a horizontal "lock" that would prevent you from moving left or right while in the air - it's a form of pseudo inertia.
This lock is not present on the original Sonic CD, but iirc, it's there in the 2011 remake.

This lock is a prefect part of the rolling design because the player would be rolling most of the time, since it's the dominant strategy most of the time.
The horizontal lock that would activate when you jump while rolling reduces the player's control - thereby making sure the player won't abuse the rolling ability.

However~
Using the insta-shield would result in breaking that horizontal lock, thereby making the player gain full control over Sonic again. This can be called the rolling jump-insta-shield combo.

As you can see, the insta-shield let's the player regain full control over Sonic again. Without it, the pseudo-inertia would stop you from going further right to exit the pit.

Because of the insta-shield(or any other elemental shield), all 3 characters, Sonic, Tails and Knuckles have their ways of breaking the horizontal lock. Tails having the fly ability and Knuckles having the glide.
But Sonic has the insta-shield - the only ability out of the above 3 to not mess up your motion - that's why Sonic is considered the most flexible out of the 3 characters.