Pianos frequently go out of tune because of many factors including change in humidity or temperature, moving and storage, as well as routine playing. The most important job of a piano technician is to provide a musical and stable tuning.
The piano has as many as 240 strings creating up to 30 tons of tension. The technician is responsible for setting each of these strings to exactly the right tension and pitch, which apply tons of force on the cast-iron plate and soundboard. The technician, in essence, is changing this massive force, and the more the tuner has to change, the less stable the tuning will be. This is why it is ideal to have a piano in a stable environment (temperature and humidity) and also have your piano tuned at least two to four times a year. Some of the concert pianos at The Conservatory are tuned twice a day!