A piano sounds best when its tuned to the international pitch standard, A-440 (basically what this means in layman’s terms is that the ‘A’ string which lies above middle C is set to vibrate at 440 cycles per second). At this pitch, the power and tonal range of your piano sounds best, especially when its also to match the pitch of other instruments in a concert or band setting.
When you as a piano owner ensure that your piano is regularly tuned (i.e. at least twice a year if not more often), you help to maintain the equilibrium of your piano strings and structure; this will also help your piano to maintain tuning stability longer after each tuning, and all wooden and felt parts within the structure to function at its designed string tension.
The longer a piano remains out of pitch, the more difficult it is for the tuner to bring it back to international pitch, and the greater the probability of piano parts within starting to malfunction.