Who is The Voices from Next to Normal and what is their importance?

On several occasions throughout the narrative, a group of singers (a chorus) sings text and music that comments, ironically or honestly, on the action playing out in the narrative. They don't necessarily have a character or identity of their own, but come into the narrative to suggest the presence or perspective of a large group of people from whom one of the characters emerges. For example, in Act 1 Scene 3, the Voices can be seen as echoing the experiences of Dr. Fine and Diana, as they deal with the large number of medications that Diana has been prescribed. In short, in some ways the Voices are simply back up singers, adding layers of musical harmony and, therefore, musical depth and variety. In other ways, however, they offer valuable insight, counterpoint and commentary on the action.