R.U.B.E is an editor that allows you to manipulate graphically a Box2D world and save it to a file. You can then load the saved file in your game/app. R.U.B.E can create any physics scene that Box2D itself can, with the exception that some joint types aren't currently supported (gear, pulley).
The main advantage of an editor like this is the speed and ease of development gained by being able to place items visually. Being able to tweak values and quickly see the results improve turnaround time when fine-tuning physical behaviors. When the editing of scenes is quick and easy, you have time to explore different ideas, creating a quality game/app.