The choice of a first-person narrative viewpoint is often particularly useful in a story that deals with an experience that is none too common for its readers. As most of us don't know what it's like to experience, say, the horrors of slavery or the degradation of drug abuse, we need to have someone in the story who has experienced such things and who can therefore provide us with a way into the stor, and the whole different world it reveals.
In Moonrise by Sarah Crossan, the first-person viewpoint is used to give us a valuable insight into the experiences of those whose family members are caught up in the criminal justice system. The narrator is Joe Moon, a seven-year-old boy whose older brother Ed has been charged with murdering a police officer. At first, Joe doesn't know quite what's going on; he's too young and naive to have the faintest understanding of what this means for himself and his family.
However, as time goes on and Ed is convicted of murder and sent to Death Row, Joe becomes more and more aware of the rampant injustice and corruption that lie at the heart of the criminal justice system. That all of this is revealed through Joe's own voice is crucial to giving us an understanding of what those on the wrong end of the criminal justice system and its manifest failures have to go through.
If the story were told through, say, a third-person omniscient narrator, then it's highly unlikely that it would've had anything like the same impact. A third-person standpoint would keep us removed from the action, from the damaging effect that Ed's conviction has upon his family. It would merely reinforce the widespread belief that this sort of thing happens to other people.
But because the story is told from Joe's point of view, in free verse, it takes on an appropriate tone of immediacy that makes us feel that what's happening to Joe and his family could just as easily happen to us.