Pianos go out of tune note by note. This might be a small thing to some, but to a student one note out of tune can make playing a piece unbearable .  Novice students can even learn the wrong intonation, interfering with more complex music study.

Keeping a piano in tune improves the longevity of your piano.  There are over 200 individual strings in any given piano each stretched to reach a specific note.  When the tension of these strings across the piano becomes uneven it divides the tension poorly bending and reshaping the frame over time.