If a piano is kept untuned for too long, or undergoes dramatic humidity extremes, the pitch of all the notes of the piano may drop significantly. The entire piano may be one, two or more notes low. When the piano is under pitch like this, other instruments or singers may have difficulty tuning with the piano.
To rectify the problem, the technician must roughly tune the piano once to "get it in the ball park". By the time he is done with the first tuning, the strings will have stretched out of tune again and the piano will then need one or more finer tunings.
A pitch raise puts a lot of sudden tension on the strings, and the piano will tend to wander out of tune again faster than normal at first, until another tuning some months later. After that the piano will become more stable with consistent tunings.
If regularly tuned thereafter, the piano will not be likely to need a pitch raise again.