Pianos have on average 230 strings that are tightened to sound at different pitches. Over time those strings are stretched from playing and environmental changes. It is generally recommended to tune a piano twice a year, usually spring and fall, to keep it in the best condition. Large shifts in temperature and humidity can cause changes inside the piano causing the piano to not stay in tune.
With anywhere from 8,000 to 12,000 parts inside a piano, very often small maintenance must be performed. Action can be regulated or adjusted to fix things like notes playing too early, dampers not raising, etc. Issues may take some diagnosing, but most problems can be fixed.
Regular tuning is crucial to maintaing your piano. Extreme changes in the tension of the piano are not good, so keeping it in tune eases this process. Keeping your piano away from extreme temperature changes will help, so avoid radiators, outdoor access or any other place that varies a lot. A constant humidity where the piano is stored can help keep the wood from swelling. Consider a humidifier for the room, or install a piano humidifier to keep the variance as little as possible.