When the instrument is not well regulated, the touch becomes heavier. The subtle playing becomes difficult to achieve and all kind of problems can appear (squeaking, notes that fail to play, etc).
With usage and time, the felts and the leathers in the piano action become compressed leading to misalignment of the mechanical parts. Dust can make its way into the system. As a consequence, the touch is affected and looses consistency from note to note, making the playing more difficult, especially in pianissimo.
The regulation process (also called servicing) consists in dusting the action (mechanical parts), lubricating and realigning those parts in order to bring back the original potential of the piano.
As a reference, it is advisable to service the action every three to six years, depending on the usage.
A serious concert hall tops up the regulation several times a year, and some even at each concert.
The consequences of a piano that has not been serviced for a long time are various: it was noticed that a beginner, especially a child, can find piano playing fastidious. The student becomes less inclined to practice and even abandon altogether.
Some people have fragility in their hands or have physiological problems. The playing in those cases can be difficult and painful. A thorough regulation would ease the difficulty.
If the regulation is still not enough, a touch weight analysis should be done to modify the touch to a convenient level to match the pianist’s requirements. See touch weight for more information about this topic.