I mean, if the entire piece is "exactly" at your skill threshold across the board, that's great. But then you also had a teacher that knew your skill threshold very well and also had an exceptional repertoire to draw on to direct you towards that piece that was just as your skill level. That's not your average music teacher.

Looping arpeggios/chord tone sequences together at progressively higher BPM, while sprinkling in stuff like string skipping, dynamics control/accent patterning, etc lets you stay close to your edge of ability and really focus on specific techniques heavily. If you're trying to make it sound good to a backing track at the same time, it also develops improvisational abilities and musical originality in a way that playing existing music won't.

I agree with all of that and was thinking that I hadn't mentioned the role of etudes and exercises directed at specific techniques or dexterity, though I guess I had classical music in mind and wasn't thinking about genres or instruments that perhaps favour improvisation over the 'other side' of a highly refined performance of a well-known piece (which still have the performer's own personality in except in a way where the listener likely already 'knows' the piece).

> That's not your average music teacher

Tangentially (or perhaps not as every other topic in 2025 is 'AI'), I was always facinated by the episodes of Star Trek: TNG that showed the holodeck and crew members being able to use that environment to essentially learn skills from the greatest performers (or even a blend) in their area.