Photographs, pull-quotes, decks, and headlines help you tell the story. Other elements such as subheads, boxes, rules, and white space help you organize the story.
For example, if you laid out three short articles on the same page, you would use rules, white space, and headlines to show readers that the articles were separate, not related.
A good layout improves readability by arranging text and graphics in a logical order. To illustrate this point, let's look at the following flyer: Example.
Every time you place a textual or graphic element on the page, you are making a rhetorical decision, and where you place that element depends on its relationship to the other pieces.
When you're writing, you organize sentences and paragraphs in a logical sequence so that readers will understand your message. You should approach layout the same way.
Just remember that page design is a flexible process. There are no hard and fast rules, just guidelines. Keep good communication with readers as your top priority, and you will make the right design choices.