Notch's root node can confuse new users. It's both the central node but it's also your active render context. What happens when you want to quickly render another texture in the background and use it as a mask, displacement map, or material texture? In this case, Render To Texture node comes to the rescue. It allows you to build a completely separate scene that gets rendered off screen and you can use that output of it just like any other texture in Notch.