Based on the details in the story, what is its primary
purpose?

It's primary purpose would be "main intent" of the story.  

Explanation:

It basically would mean these things : What is the author trying to inform, persuade, or make out of this story? If there are "details" in the story then the authors "purpose" is to describe those details. Hope this helps. <3

Depending on the story you are talking about, the primary purpose is the main objective that the author has when writing his piece of text. In a story the most commons objetives for writing is for entertain or share an opinion telling a story. You can also identify the primary purpose by finding the lesson you obtained for writing the story.