IMAGINE a piano that never had a tuning fork or other tuning device applied during the tuning process; and, only a single person's trained ear to make sure it was in tune. Worse yet, imagine a committee of three to decide how each key was to be tuned. It would, then, all depend upon the ear of the particular person doing the tuning. (Could it be Beethoven? ) A piano's tuning would vary based on ONLY the subjective judgment of an individual tuner. There MUST be an objective, dependable reference and it cannot ONLY be a person's unaided subjective ear. OTOH, the tuning device needs the trained ear to do the actual physical adjustments to the instrument.