Not everyone tunes their piano equal temperament. I use an alternative tuning ebvtiii which I think is a better compromise. There are a ton of well-tempered tunings with various compromises. I would like a piano that has 15 keys per octave so I can tune it in quarter comma mean tone. (i've seen pipe organs done this way, but never piano.)

Wikipedia has an entry for well tempered as well https://en.wikipedia.org/wiki/Well_temperament