A piano holds tens of thousands of pounds of string tension, and that tension slowly relaxes. Add the humidity swings of a North Texas year and the pitch drifts. A yearly tuning corrects that drift before it gets far, which keeps each tuning quicker and the piano more stable.
Tune every six months if the piano is used for daily practice, taught on, or played in public. Serious students and performers notice pitch drift long before a casual player does, and a piano that is out of tune is harder to learn on.
A piano left for years sinks well below standard pitch. It can still be brought back, but it needs a pitch raise first to lift the tension, then a fine tuning to set it. Waiting does not save money in the long run, it just adds a step.