Cross Training is pretty common among good athletes. A runner may run races, but vary their running workouts with barre classes and strength training as a way to make their body stronger as a whole. This then helps their running.
Learning to tune pianos has been a good exercise in cross training for me. Pianists never really have to rely on their ear for pitch, because they have no control over it. My sense of intonation was really bad. When I accompanied other instruments, I could at times tell if they were out of tune, but rarely in which direction (flat or sharp). When I started learning how to tune pianos, I knew I would have to spend considerable time teaching my ear to hear these things accurately.
Listening for tuning is completely different than listening for playing. When tuning pianos, you are listening to the vibrations (or beats) between the two pitches. It may sound like a weird concept but I found it actually quite easy. When I listen for tuning, I listen beyond the note for the specific pitch and interaction of the strings. It is a very analytical way of listening. (And I LOVE things that are actually calculable like that. I like having right and wrong answers). When I listen for playing, I hear all the notes together and listen for balance and tone. This is a more artistic way of listening, very subjective, and definitely not my strongest skill.
There's good news for anyone struggling with listening. Time is on your side. Playing music for 20 years has made my artistic listening a lot better, especially my five years of intense training (aka college) and the three years I have spent afterward unpacking everything I learned. Tuning pianos for a year has also made my analytical listening better.
So back to the cross training. I've seen two instances where learning to listen for tuning pianos has helped me in teaching and accompanying!
I accompany college students for their lessons, recitals, and juries. This year I have finally been able to help them tune to the piano! I have accompanied for years, but was never much help when it came to listening for intonation. Now there is a significant change in what I hear and how I can help them.
The second instance was when working with one of my piano students. He was working on recital pieces, one of which includes a cellist. I can coach my students on playing together, rhythms and note problems, balance issues, musicality, etc, but I have never been able to coach the assisting instruments on intonation. Until now. I was able to help the cellist match the pianist and give him pointers that he might not receive without a private teacher.
These might both sound like little things, but for me they are huge! For most other instruments, the musician has to learn to listen for intonation from the very beginning. For piano, this is just a big black hole, which until now I hadn't bothered to fill. I am glad to see that learning to tune pianos is strengthening my other piano related skills.