A hammers work is to strike the strings and produce a good sound as well as pleasant tone. Most people purchase their piano with a clear concept of the type of tone they are after Some purchase their pianos second hand and have the piano re-voiced to suit their taste. Some have an inherited piano who's tone is "honky-tonk" and needs re-voicing.

With the pianos hammer set, after some years use, the woollen coverings "striking surface" hitting the strings,the felting will almost certainly have "packed up" ( that is, become tighter )
and become harder in texture resulting in a harsher hardened brighter tone. This may be agreeable but to most it is not. Wear in the form of deep grooves depending on previous use and or environmental damages caused by household fumes containing greases. This all contributes to the hammer felts changes in the original tone of your piano.

Often tone becomes irregular from note to note and needs to be adjusted individually.
Some notes are smooth toned followed by louder brighter notes. This can be due to pianists who favour one key signature like say F major more than others. All the notes related to this key become harsher while in say C# major most notes still sound smooth as when the piano was made.
In some cases, the whole hammer-set needs "re-toning" or "voicing " depending on what has occurred, I prefer to
disscuss with the owner of the piano how they should like the piano toned and to be present for initial toning of the middle register. After this my work is to align the tone of the notes evenly across the pianos compass ( range )
So that when played chromatically the tone is a constant. Sometimes this may involve just surface re toning and others require deep toning or even chemical treatment to brighten an overly dull toned piano.
Re-toning an upright costs from $75 to around $250 depending on what is required.
Re-toning a Grand costs from$125 to around $500 again depending on what is required