MAINTENANCE PRESERVES THE VALUE
Regular Tuning preserves the value of a good quality upright or grand piano. Because the instrument responds to climate changes as well as use, it will only keep its original condition if you have your instrument serviced regularly. The convex/concave shape of the spruce soundboard was design-engineered & manufactured for the constant string tension of A-440 pitch. When the pitch falls considerably, the dimensional shape of the soundboard changes somewhat. Whether this becomes permanent or not, it does lessen the tone quality and sound volume of the instrument, from that which it is capable of.
Severe falling pitch brings misshapen piano wire from the bridge, agraffes and pressure bars into the speaking length of the string, causing "wild strings" and "false beats" to become a permanent sound that cannot be corrected by tuning.
Climate changes (temperature and humidity changes) and mechanical stresses on the piano, these all have an effect on the tension of the strings. So over a period of time the piano goes out of tune. Your piano should be tuned at least ONCE A YEAR. This preserves both the value of the instrument and the musical enjoyment.