In "By the Waters of Babylon" why does the author include the narrator's out-of-body experience?

The first purpose is to explain what exactly happened in
the great burning, and to help the John (and by extension, the reader) understand that
it was mankind that destroyed themselves, and not some strange event caused by Gods.  It
was simply a literary tool, and a handy one, to help explain what had happened, like a
flashback in a movie.

A second purpose would be to remain
true and faithful to the storyline of a tribe that sends its young men out on spirit
walks.  In many cultures, the young men participating in these vision quests, or spirit
walks, did have actual visions or spiritual experiences that revealed great truths to
them.  The entire story centers around John, a young tribesman, who goes out in search
of wisdom and learning in his tribe's version of a vision quest.  The vision that he
received was his answer, and he was able to go home and tell his father about it.  His
father was able to conclude that was a true vision, and one that John was meant to have;
this was all part of their culture, so the vision that the author describes stays
faithful to it.

I hope that helped; good
luck!