After several years of playing the felt on the hammers will wear down, and grooves will form on them from striking the strings. When the grooves become too deep, the resilient shape of the hammers is lost and the tone becomes less pleasing. This happens so gradually, that the pianist will probably hardly notice the change.
To remedy this, the technician will sand the hammers to the original shape and remove the grooves. This process can dramatically enhance the tone, restoring it to its former richness.