Very often, things are evenly arranged around something big in the center. Asymmetry is a good break from the norm. I used the golden mean to compose the photo of the frozen leaf. Although the background is graphic and is important to the photo, there is only one major element, the leaf. There is probably no stronger graphic tool than a diagonal line in a vertical photograph.
Diagonal lines add tension and make images appear more dynamic.
It may be that we associate diagonals with instability. Take, for example, the Leaning Tower of Pisa. It is only a gentle diagonal, yet it appears very unstable. Photographs of sailboats making turns and cyclists rounding corners are good examples of strong diagonals. The lines of the tiles are also diagonal.