Piano strings loose their beautiful tone over time. They become stiff and literally do not vibrate as well. They also become brittle and more likely to break when tuned(this is especially true if the piano has not been tuned in many years). If your piano does not have the pleasing, rich tone it once did; if strings are often breaking during tunings; or if your piano has not been tuned for many years your piano probably needs to be restrung. This can often be done without moving the piano.