According to classical design theory, graphic elements strengthen an image and manipulate the viewer's attention and response. Although content is the priority of the photojournalist, graphic elements help tell the story. Photographers who know that diagonal lines add tension or that single points draw attention are more likely to produce better images, more often.
But it isn't enough just to train in the principles of design. Photographers have to internalize them. They have to make them part of their subconscious thought process and make design decisions intuitively. For photographers, decisions must often be made in less time than it takes for elements to be recognized. For most photojournalists this means it just looked right.