In this case, the purpose of the authors with this passage is to emphasize the rivalry between Jack and Ralph, this is shown as the author explains the differences between Jack and Ralph ideas about survival. This passage shows Ralph would us commonsense, logic and civilized principles to survive through "the world of longing" (what is real) and the "baffled common sense". On the other hand, Jack would use more barbaric strategies such as "hunting" or "fierce exhilaration".
These opposite points of view are confronted during the story as Jack becomes the antagonist and a barbaric leader. While Ralph attempts to be the civilized leader of the group.
It is the theme of the story :)
