Scotty Barnhart advocates transcribing jazz solos in real time because he’s heard too many note renderings that lack the spirit, which is really the point after all. The improviser that sounded so good was to some extent thinking about the notes but in the moment was feeling it in the lips, diaphragm, fingers, ears and eyes. That’s where the music lives.
As I’m learning piano charts from sheet music, it’s pretty obvious which ones were improvised and then written, and which were composed on paper and left for the pianist to master. You can feel it in your tendons. It ultimately lets you see into the composer’s mind, to really get what they were thinking and experiencing while writing.
It also makes it easy to work out fingerings, phrasing, dynamics, and generally the musicality of the piece. Maybe you can learn that by studying sheet music–I had a composition professor who never even listened to the notes he put on the pages–but following in someone’s fingersteps connects you to the piece directly.
I’m learning lessons that I probably wasn’t ready to learn when I was trying to learn them. I find they’re showing up in other areas of performance. I haven’t been writing while pursuing this project, but I’ll bet it informs some of that work too.