Does a silent keyboard not defeat the purpose of finding out
how to play a piece in order get the best tonality
and finally achieve the best sound ?
Actually not. I thought so for very long, but then I found out that I learned more control if I tried to vary my playing, that is, to play "wrong" (not wrong keys or wrong fingering, but everything else "wrong"). After doing so for a while, I could then play the piece exactly AS I WANTED.
After all, you work out the musical details in your head. There's where you really create the music.
Or you try to play perfectly every time and put all the brain work at your fingertips, so to speak. Then one day you have to play on a totally different piano, and get shocked by the difference in sound and feeling, and your playing breaks down. Been there, done that!
So practicing on a silent keyboard is great when you just have to learn to hit the right keys in the right order. Your ears will not get tired. You can develop your idea of the music without getting disturbed by the noise ...
Practicing while watching TV or reading a book is, if you ask me, totally worthless and I don't care if This or That Great Pianist has done it. It is still a stupid habit. Mindless practice is worse than no practice at all. You will end up giving mindless performances.