In "By the Waters of Babylon" how is the first-person point-of-view an advantage to the reader?
The first-person point-of-view is incredibly helpful to the reader in this story, because John lives in a world that is not our time period, and in a culture that is very unusual and foreign to us. John is able to explain, through his narrative and thoughts, many aspects of that culture, as a first-hand witness who lives it each and every day.
Also, because John is a priest, he is privileged to know things about their world that other members of the tribe would not have; getting things from his personal perspective opens us up to a lot of information that we would not have known otherwise.
The first-person perspective also allows us to understand John's thoughts as he goes on his long, lonely spirit walk to the Place of the Gods. Without the first-person narration, all we would see is John walking, and know nothing about his dreams, his fears, his hopes, and his own personal interpretations of things.
Lastly, the first-person narrative makes it feel more like a story that is told and passed down from generation to generation. This is a tradition of all ancient cultures; they didn't write things down, they told the stories to one another. The first-person narration makes us feel like we are sitting around the campfire with him, listening to him tell the story for the first time. We get his spin on it, his wise perspective, and his fresh eyes as he journeys, and all of it is told as if he were speaking directly to us. That makes it more personal, relatable, and also in sync with the culture that he came from.
I hope that helps a bit; good luck!