Voicing is an art to enhance the color of the sound.
The sound your piano makes should be/could be also adjusted during maintenance.
Under heavy use or even with time, not just the action can wear out, but the sound, too. Voicing is a part of tone regulation. Within the capabilities of the parts built into your instrument, and the size of your piano (since longer strings always mean more “body’), voicing will make your piano sing again. Voicing can adjust the sound in many different ways. If your piano is too loud, it can be “voiced down”, or if it is too soft, it can be brightened up again, or if the sound is uneven between different registers it can be changed, too. Furthermore, the articulation of each hammer can be adjusted to give you the warmth and the richness of the tone of your piano.
Needling or sometimes enforcing the hammers, and in case of a restoration, choosing the right quality hammers or the hammers with the proper tonal characteristics, can make miracles with your piano.