Including js libraries like jquery, underscore, angular etc, you can also use old versions of these libraries to test backward compatability.
Including any exernal resources
realtime collaboration - you can collaborate with another user on your fiddle and edit it simultaneously. In the screeshot below we see two different browser sessions, and each session the user can see the other users mouse icon
IDEone lets you write compile and run code in many different languages. Sometimes its more convinient to test stuff out with this online tool then setting things up in your desktop IDE. The screen has 3 parts,
stdin (your codes input that goes to main(String args) in java's case)
stdout (where you see the result after you run or "ideone" your code)