Pure Data's domain is digital signal processing, with an emphasis on sound. It's good for prototyping because one edits the signal graph, GUI, and event-triggered scripts in the same visual window using the same event loop.
So in a way it's good for building idiosyncratic little "musical hex editors":
I say idiosyncratic because when you can connect anything to anything else, weird things start to happen. (For example, adding two more boxes to the diagram to control the crossfader with the pitch of someone's voice.)