Stephen Vincent Benet would have had two main reasons for writing this story. First, he clearly wanted to write something that would be entertaining and suspenseful. Part of the purpose of any writer of fiction is to entertain. A writer who only wanted to get a point across could do so in essay form rather than through fiction.
That said, Benet clearly wanted to make a point as well. He wrote this story in large part to warn the world of the consequences of modernization and the potential for destruction that lies within all of us. We can see this in at least two ways. First, we can clearly see that the previous civilization in this story, the one that had inhabited what was clearly New York City, destroyed itself through war. Although Benet was writing before WWII even began, he feared that technology was advancing rapidly enough that people would one day be able to destroy whole civilizations. Second, we can see that there is the potential to make the same mistake within John himself. He clearly sees his people as superior to the “Forest People.” This can be seen as an instance of the sort of racism and nationalism that had brought the world to the brink of disaster in Benet’s time. We can see it at the end of the story when John says “We must build again.” It implies that the same cycle of civilization and destruction could begin all over again.
Benet, then, is warning us that the combination of our human nature and our technological capabilities is very dangerous to us.