I'm an amateur and have historically only learned pieces that I love hearing or that I think would be fun to play. In my youth, I used to play scales and other small exercises as directed by my piano teacher to build up technique, but with time now being more limited, I now only work on repertoire I enjoy.
I've long loved the Chopin Etudes but found them pretty much impossible to learn/play on our VPC-1 digital piano. Now that we have a grand, I've revisited Op 25 no 1 (Aeolian harp) and surprisingly found that the real acoustic action has made a tremendous difference in the feasibility of learning this piece.
I've been working on it for only about 2 weeks, but I can definitely feel my strength and technique improving. I initially thought my hands were too small for the arpeggios, but each day it seems more achievable. My rolls and jumps are getting more accurate, and my finger strength is improving. My forearms burn after each playing session!
What has surprised me (but perhaps shouldn't have) is that my improving finger strength and control is helping with other pieces, as well. I was previously stuck on a difficult section in Liebestraum No 3, and after revisiting it last night, I found that my slightly improved finger strength/control really helped!
I'm honestly surprised that just 2 weeks of working on this Etude has made a measurable difference in my technique. I'm curious, what pieces have you found to significantly improve your strength or technique?