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
Notes From the Piano Bench
Questions and Answers and updated factoids concerning pianos in the 21st Century.