To some degree, this choice reflects the author's interest in crafting a kind of detective story—one in which the reader is given the opportunity to work out the answers. This choice also permits the author to elicit emotion and view our world from a different perspective. Let's consider these functions in more detail.
What is the setting of this story? Who are John's Hill People? Where do they live? What's the mystery of the Place of the Gods? As the story proceeds, we interpret clues that John can't decipher. If Benét had told the story from the point of view of a more knowledgeable actor, we would have less opportunity to work out these clues for ourselves. The narrator would have done that for us.
Benét's choice of narrator also has emotional effects. By telling the story from John's limited point of view, Benét is making use of dramatic irony—a literary device by which the reader's understanding extends beyond that of the characters in the story. As we decipher the clues, we realize the civilization of the gods was really that of 20th century America. This insight fuels our involvement in John's journey: We want to see if he will figure things out.
We understand, for instance, that the religious injunctions (against entering ruined buildings, handling metal, or visiting the Place of the Gods) are poorly-remembered folk warnings about real health hazards. When Benét wrote his story in 1937, nuclear weapons hadn't yet been invented. His audience was acquainted with chemical weapons, though, and understood that they could leave dangerous residue behind. Will John discover the truth? We know that we could help John if we could speak to him, and this may increase our personal involvement in his journey.
Dramatic irony can also lead to amusement when John gets things wrong, as when he wonders if the subway tunnels were used to house slaves. It may also make us feel a sense of urgency about the story's ending. Will John's people, ignorant as they are, make the same mistakes that led to the destruction of 20th century civilization?
Finally, a naïve narrator provides us with an opportunity to see our world from a different perspective. A middle-class apartment appears to John as a "place of great riches." Grand Central Station looks like "a mighty temple." Things that we take for granted are great wonders to him, and this may heighten our sense of loss and perception of the waste of war.