Mark Johnson of the University of Georgia writes:
One of the challenges in photography and, by extension, photojournalism is compressing a three dimensional world into two dimensions.
Another of the challenges we face as shooters in establishing a visual hierarchy. The shooter brings all of her senses into play when they're in the field. They hear things, smell things, sense things, have a sense of the timeline as well as see everything around them.
When a reader comes to that image, none of that information exists - and this is why picture editors are so important. The photojournalist knows what their photo is of and about; the reader has no idea.
The photojournalist will see subtle little things that the reader won't because they have a memory associated with the photo. The reader will see poor composition, or a cluttered background and not see the proverbial tree through the forest.