Large changes in humidity are your piano's biggest enemy. These changes are by far the number one reason your piano goes out of tune. Extreme levels of humidity can also damage your piano in other ways over time.
Overly dry air that comes with winter weather can cause wood parts to shrink and crack, especially the soundboard which is what gives your piano it's voice. Glue joints can fail and tuning pins may become loose.
High humidity creates it's own set of problems. Cabinet parts swell and bind. Keys stick and strings can rust over time.
The Dampp-Chaser is a humidity control system that when installed discreetly inside your piano, keeps the air inside close to an ideal of 45% humidity. Doing this prevents the piano from experiencing large swings in pitch with the seasonal changes and will dramatically improve tuning stability. It will also help protect your piano from most of the above mentioned problems encountered with extremes in humidity.
Pianos that benefit most from these systems are pianos that are inherently unstable and go out of tune more quickly and severely than we would expect, and institutional pianos such that are found in schools and churches where the temperature and humidity are not controlled during long periods of non use.
I sell and install these systems and I hope you will feel free to call me for more information as to whether your piano could benefit from climate control.