Every piano has its own distinct sound or "voice". In many ways, that's how it was built and that is its personality. But over time, the more a piano is played, the more the hammer felt that strikes the strings wears down and compacts.
This hardening can cause the tone to become too harsh or bright, and it's difficult to produce a wide or even range of expression, no matter how much you try to play using dynamics. In other situations, the piano may seem too dull or lifeless for your taste.
Either way, a technician can treat the hammers in various ways: by aligning, shaping, careful needling or hardening the hammers, in order to get the tone you're looking for. Keep in mind, however, that a piano has to be finely tuned and regulated before it can be voiced.