I've seen several online code tools that allow you to compile, run, and even debug CLR code online. What's special about this one? The back-end Roslyn interface looks interesting though: http://msdn.microsoft.com/en-us/roslyn
Compilr, Coderun, and Ideone all look like great in-browser replacements for a full blown IDE. The builds even take as long as a Visual Studio would. They're great for when you want or need the full IDE experience.
But there are many times when a developer might not want that experience. Maybe you're brainstorming a new implementation for an algorithm and you just want to get an idea of what it might look like. Maybe you're in the middle of a presentation and someone in the audience asked you a slightly off-topic question, and you don't want to spin up a new instance of an IDE just to help them understand.
This tool is for those situations like those.
Edit: here's the one I've used before, complete with integrated debugger: http://www.coderun.com/