Every five (5) to ten (10) years, more often for heavily used pianos, you'll need more than just a tuning. Pianos, like cars, need modulation to facilitate a return to the manufacturers' specifications. Returning a piano to the specifications when it left the factory, is called regulation.
With approximately 4,500 moving parts while playing a piano, these parts eventually go out of adjustment. Wood parts, such as the 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.