Why was Dionysus put in charge of Camp Half-Blood?
My theory is

1) if the main idea is to show the sequence of events, a cause-and-effect style of writing will be used.  2) if the main idea is to show why one product or idea is better than another, the basic points of each side will be compared and contrasted.  3) if the main idea is to present opposing viewpoints, the pros and cons of each side will be discussed.  4) if the main idea is to draw special attention to an important cause or event, various examples will be provided.  5) if the main idea is to define a certain term or concept, various examples and explanations will be provided.