With pianos there are actually two things going on. They're tuned approximately to 12-tone equal temperament, but also the octave is stretched due to the inharmonicity inherent in the strings. (In the middle-range the strings are pretty well behaved, but in the high treble they start acting more like metal cylinders than ideal strings, and the bass has wound and double wound strings to similarly mess things up. The result is that the harmonics of piano strings aren't exactly multiples of the fundamental frequency, and so the tuning is stretched so that harmonics of lower notes more closely match the fundamentals of higher notes.)

https://en.wikipedia.org/wiki/Piano_acoustics#The_Railsback_...

Well, just intonation generally sounds better to humans. A person playing violin would tend to naturally migrate to JI voicings of intervals whether they're consciously doing it or not. So, a tool that tells you whether you're "in tune" or not should ideally not tell someone they're out-of-tune when if they play in perfect JI.

On the other hand, if the goal is to just be in the general ballpark and accuracy of +/- 15 cents or so is just rounding error, then ET is probably fine. (My own violin skills are pretty rudimentary, so ET would be good enough. But if I were better I'd probably want a JI tuner.)