Made by Piano Life Saver, these systems work to keep the humidity level in your piano as close to 42% year-round as possible. Fluctuation in humidity is your piano's worst enemy as well the reason that your piano goes out of tune.
Excess humidity can cause strings and tuning pins to rust and the moving parts of your piano (the action) to become sluggish or even non-functional.
If your piano gets too dry, the soundboard can crack, bridges and strings are effected, and the piano action can feel loose and sloppy - even unglued.
A heating element (2 in grands) plus a water tank are installed. A humidistat controls these components and tells them when to turn on and off. Aside from adding water in the Winter, these systems are maintenance free and are practically invisible.
If you have a newer HVAC system with humidity control built-in and you keep your windows closed most of the year, probably not. If your piano is in a basement, an older home, goes out of tune more quickly than you'd like, or is in a very large room (like a church sanctuary) then you will definitely benefit from this system.