Cycles is amazing, the principle BSDF even Better, But when you want to go to the last 10% of awesomeness you need to go fine tune. That's what this node is about.
It will give you the full control over the Index Of Refraction and even bend the reality for artistic purpose.
Simple, fast and self-explanatory you'll be able to enjoy the freedom to create advanced shaders.
Enjoy it...for free of course!