This site/tool/book is fascinating. The editor interface reminds me of EA’s Instant Music on the Amiga, software I spent many hours with as a teenager.
There is a moment in every musicians life when they realize they want to understand more. Why do certain chords fit together easily, and others not so easily? Why do certain notes seem to sound “perfect”? How can I get from this chord to that chord? How can I create a great sounding melody?