Pianists often speak of piano tone as a fixed quality of an instrument. “This piano has a harsh tone,” they say, as they might say “This piano’s a seven-foot grand.” But tone is affected by many factors: the “crown” of the soundboard, the quality and condition of the strings, the shape and texture of the hammers. And whether the instrument’s in tune.
Unisons on an out of tune piano have weird beats and may sound twangy or sour. Intervals and chords will misrepresent the composer’s harmonies and sonorities. Thus, piano out of tune means tone out of whack. Piano in tune doesn’t guarantee beautiful tone, but does mean the tone will be the optimum allowed by those other factors: soundboard, strings, hammers, and so on.
If you’re evaluating a piano for purchase, have it tuned close to when you’ll hear it; on the same day, for sure. Ditto for an instrument you’ll perform on. If your performance will be recorded, have the tuner “on stand-by” (staying till intermission) to touch up the unisons that go out in the first half of the concert. 
