Just as we need unity, we need variety. While too much variety results in chaos, too much unity results in boredom.
Artists work to incorporate both unity and variety into their work.
Look again at Seurat's painting, illustration 10-1 on page 116. Each of the figures is dressed differently and posed differently, even though they all face the same direction along the same shoreline. There is one boat on the water that is not a sailboat. Only one of the smokestacks in the background is actually spewing smoke. The trees are all different sizes and shapes.