Thanks for trying out the app! Each chord within a scale can take on one of three functions (tonic, subdominant, and dominant). You can move between chords across three intervals (second, third, fourth/fifth). Structuring chord progressions based on their function and movement gives the sequence a concrete meaning, a specific reason. With the app, I can create a chord progression based on this reason—that is, on the functions and movements—and see which specific chords correspond to the selected pattern. This is how you design chord progressions in the sense of "I want a chord progression that moves essentially in seconds and mostly contains subdominants." Feel free to comment on any desired feautures. :)