Why tune your piano?
The obvious answer is to keep it in good tune so it is a pleasure to play.
What you may not know is that the tension on a piano is so great, if it is not kept continually balanced, damage can occur to the piano. An annual tuning keeps the tension across the treble, tenor and bass for improved stability. Any good tuner will tune to the very highest note and the very lowest note to keep a proper crown in the middle section. Depending on how new the piano is, how much it is played or the climate it lives in, you may wish to tune it twice a year. Professional players often tune it 3 to 4 times a year. For the average piano owner, once a year makes sure nothing wanders too far and gives the technician an opportunity to adjust mechanics as needed.