It's simple, easy to use for developers that are looking to test their code right away without having to install any additional things or tools.
Sometimes it doesn't work when you want something different from what's shown in its examples (e.g., adding new functions). Also I think there should be more features which can make my life easier than using jsbin as an editor only! My goal was just testing small chunks of codes/scripting - writing some quick prototypes etc..
but now with this toolβ¦