There seems to be much said about regulating grand pianos, but verticals need it too - they sound and play better, and also help the piano last longer. Here's a pdf from the Piano Owner's Repair Guide that will help you better understand what it is all about.
Pianos go out of tune primarily because of changes in humidity. Tuning can be made more stable by installing special equipment to regulate humidity, inside or underneath the piano. There is no evidence that being out-of-tune permanently harms the piano itself. However, a long-term low-humidity environment may eventually crack the soundboard and warp keys and other wooden parts. In particular, pianos located in arid climates and otherwise very dry rooms (for example, a cold climate with an extended heating season) require special attention to humidity control.
From Wikipedia, the free encyclopedia
One of the things I learned as I went from piano player to piano technician is that here is more to piano maintenance than just tuning. For instance, as a piano is played, the strings cut grooves into the hammers. This will start to give the piano a harsh tone, and the player loses control over the dynamic levels...it goes from play soft to loud, to loud and louder.
Well, anyway, here's a file you can look at to get a better idea of what groovy piano hammers are and why they need to come out!
Why the Hammers on My Piano Need Filing
To better understand how humidity affects your piano and a means to control it, take a look at this video by Dampp Chaser. It will also show you how the Dampp Chaser works.