Creating depth: the photographer angled the camera in a way that shows the length of the leaf tunnel. By doing so we get a sense of how far back this goes. To add to the sense of depth the photographer positioned a person to the bottom right corner to let us compare the size of the person to the size of the tunnel.
Avoiding mergers: the photographer took this photo at the time the olympian was in that exact spot to avoid her merging with the audience. If the olympian were in front of the audience at the time the photographer shot the picture the photograph might have looked busy and messy. Avoiding mergers helps us focus on the main part of the picture, in this case, the olympian.
Background: the photographer blurred the background of this picture to help focus on the soldier. By doing so the photographer makes the soldiers facial expression the main focus of the picture. If the background were not blurred it may have been difficult to focus on the soldier and understand what the photographer is trying to say with this photo.
Viewpoint: this picture has a unique perspective in that it shows us our President in a way we rarely see him. This has a deeper more personal feel to it, by the expression on his face, his composition and the sun in the background we see a meaningful individual instead of a politician. If the photographer had taken a more traditional approach to a presidential portrait, front facing, looking friendly, perhaps in front of an American flag, we might have thought it nice but otherwise have been indifferent towards it, in this picture we really see the leader of a nation.