My grandfather taught me how to tune piano’s as he did for 60 years once he discovered I had perfect pitch. I have old style tuning forks as well as my Grandfathers 1877 Marble handled Tuner.
With tuning by ear, you tune to the ambiance of your room/home. New digital tuners will tune to the exact correct Htz for a note, BUT, many sound flat and even out of tune due to the type of room the piano is in. I tune by ear as man times you want to tune to the best sound where the piano fits the sound of the room. This can ONLY be done tuning by ear.