Voices could mean sounds that imitate human voices, or it could mean sounds different than the basic piano sound on a DP (these could be various horns, strings, synth sounds, etc).
In either case, I'll try to answer what use they are to me; for other people, these voices will have different uses or maybe none at all.
I've recorded a number of easy listening standards, and find that some of my arrangements stand by themselves (piano solo), whereas others just don't, and need "something else".
With the "others", I often score in orchestration using violins, various brass instruments (these don't usually sound too realistic), or even imitation human voices.
I add these secondary tracks using midi (note that the piano track is recorded as midi, so it can be combined in a sequencer program with the other instruments.
[As an aside, I'll add that my sequencer will play these tracks together (with the sounds generated by my DP) so I can judge what the "orchestra" sounds like before it gets rendered to a wave file.]
Mostly though, I will add percussion and bass tracks. I would also consider these as "voices" in the broad sense.
With 900 plus "voices" that are grouped as Piano, Organ, Guitar/Bass, Strings, Sax/Brass, and Voice, I have a confusing array of choices of voices.
Essentially what I'm doing is trying to pretend that I am an arranger.
It's fun. With 950 voices I've had a lot of fun.
And you'd be surprised how good some of these "voices" sound with piano. One is Warm Pad (which likely comes from the synth background), and is quite useful. I've even used "Rich Choir".
Hope this helps a bit.
Glenn