I designed these for myself and my students. The D12 has a different musical key on each face, the D4 has a different basic chord quality (so major, minor, diminished, and augmented), and the D6 has different seventh chord qualities (major, minor, dominant, diminished, half diminished, and major-minor). So if you're practicing seventh chords, for example, you'd roll the D12 and the D6 and then play F#7 (assuming that's what came up). The D12 is also useful by itself for practicing a chord progression in different keys or just as a creative tool for generating ideas.

I have a YouTube channel where I upload music lessons, mostly centered around music theory concepts. I show off/talk about these dice a good bit in this video (although I also talk about my channel in general and my Patreon campaign, so be warned) https://youtu.be/AGsRm26uBwU

Oh and if you're not familiar with the different symbols: Triangle - major Minus - minor Circle - diminished Circle with a Slash - half diminished (also known as minor7 flat-five) Plus - augmented

And of course you can always come check out my YouTube channel if you want to learn more :)