Concert pitch A440 (A above middle C vibrating at 440 times per second) has been universally accepted as the pitch to which all instruments should be tuned. It ensures that when instruments play together, they will all be in tune with one another.
Pianos are designed and built to sound their best when tuned to A440. So when your piano's pitch drops due to lack of tuning, the quality of tone suffers.
Also, if a piano is not tuned for quite some time, its pitch begins to slip far enough away from concert pitch that it may require a pitch raise in order to achieve a stable tuning.