Imagine you are programming in a computer which has no "PLAY" key and you want to create an event triggered by the "PLAY" key in a TV.
You need to know the keycode for "PLAY".
They are references for javascript keycodes on the web (the first result of google does not display info for multimedia keys, though).
Anyway, a keycode list in the manual entry for the keyboard object would be nice.
Also, some special info on keycodes for non-standard entries should be nice, too. Such as "The ok button on remotes sends ENTER".