Every eight (8) or ten (10) years, maybe more often depending on the use the piano gets, it needs more than just a tuning. Pianos, like cars, occasionally need to be tuned and brought up to the manufacturers' specifications. Bringing a piano up to the manufacturers' specs is regulation.
There are approximately 4,500 parts that move during the playing of a piano. All of these parts can, and do, go out of adjustment. Wood parts, like keys, swell and shrink slightly with every seasonal change and become out of adjustment. So do all of the other action parts, also made of wood. Felts under the keys, and in the action, get crushed down during playing and accumulate dust and dirt from just sitting for years.
There are about twenty (20) things per key which can be adjusted to make the note play better. Keeping the correct relationship between every part will prevent unnecessary wear, and will make the action feel right. Each key will operate smoothly and evenly throughout the entire keyboard. No more loss of power, or slow repetition, or double striking and bouncing.