I plan to use it to tweak values during game development, so I don't have to edit/reload every time I tweak a number a tiny bit.

The liveness of something like this can be rather useful actually.

I wrote a not-as-fancy "canvas explorer" with a live updating image that you can save snapshots of. [1] Wrote this just yesterday to let me programmatically make images for use elsewhere (I suck at drawing tools.) I'm already enjoying the "liveness" of it and the fact that all images are remembered in localStorage. It's really cool that this is so easy to do with JS.

[1] http://srikumarks.github.com/demos/canvas_explorer/

Plain and simple (if you could ever call it that) exploratory programming? The sort that yields stuff like Jared Tarbell's stuff http://levitated.net/daily/index.html and many Chrome Experiments, etc.