I think that the book ends this way in order to end on a somewhat hopeful note. It also reminds me of a Dickens book because it tells what happens to everyone. It wraps everything up in a nice little bow. We know that Lewis is dishonorably discharged from the army, Hubbard goes to Canada, and Keith gets a job at home as a security guard. By focusing on Phillip, we get the hopeful aspect because Phillip is introspective. He goes to Vietnam, he sees, he thinks, and he returns a different man but ready to live his life.
I believe that the choice of narration is provided so that readers can gain as many different points-of-view as they can. It is always good, or at least in my experience, to "hear" different voices as I move through a text. Multiple voices keeps the text "honest" to a certain point.
There are multiple points of view. The first chapter is told from a third person omniscient point of view, and the next three chapters and the final chapter are told from the point of view of Philip, who describes his time in the army first hand. There is one chapter in between (the chapter that describes the encounter with the prostitute) told from the point of view of Lewis.
Thank you so much for your help.
I think that Wolff did it this way because he wants us to see it from the point of view for the different chaeacters not only one character.