Wow, this is probably the most intuitively enjoyable music tool I've ever used.
I'm not a musician and I know very little about what makes music good, but playing with this tool felt like I was hearing a better version of my own imagination. Like those scenes in movies where people can suddenly play music and have no idea how they're doing it.
PG Music's Band-in-a-Box still exists, but way back in the Windows 3.1 era it already had a "snap to pleasant notes" feature if you wanted to play along a set of chords on a MIDI piano. It could also play chords in the style of famous musicians along with your never-ever-bad notes! I always switched it to the "Erroll Garner" preset and hammered away.
That's the magic of music theory: it constraints the space of all possible invocations of an instrument to those that are theoretically correct. Our 'random input' is fit upon the closest match that yields a music-theoretical correct outcome.
Western music has certain "rules" that our ears are used to. If the computer limits variation/random-ness to stay within those rules, it will sound at least "pleasant", as the bot won't let you drive outside the lines. However, the real artistry is knowing when and where to break the rules. A tune that sticks to all the rules risks sounding stale or ordinary. You gotta know where to spice and spike it to move to the next level.
I'm not a musician and I know very little about what makes music good, but playing with this tool felt like I was hearing a better version of my own imagination. Like those scenes in movies where people can suddenly play music and have no idea how they're doing it.