The Importance of Regular tuning
It will seem this advice coming from a piano tuner is biased and perhaps even self serving so I will attempt to describe in a simple fashion how regular tuning is in fact an important practice. Stability. Stability is the object every piano tuner strives for and is attempting to achieve in your piano. Stability of pitch so that wherever the piano tuner leaves the tuning pins and strings, they stay. The smaller the adjustment, the more stable it will be. A piano that is grossly below pitch and out of tune requires large adjustments. These large adjustments come as a shock to the piano and the piano reacts in like manner. The piano reacts to the shock of this huge pitch change by trying to stabilize itself. The change in pressure on the wood and tension on the strings eventually do stabilize themselves but not necessarily where the piano tuner intended them to be stabilized. This happens after the tuner has left and is long gone. It may take a couple days, but it does happen and the piano usually sounds better than it did, but not as good as it could.
In the instance that a piano is tuned at best 4 times a year or at least once a year, the tuner can make smaller adjustments allowing the piano little to no reaction. Thus, the pianos pitch stays where the tuner intends it to stay.