When studying composition, there are many "rules" or guidelines to consider to strengthen an image. Concepts include the rule of thirds, symmetry, leading lines, etc. The S-curve can also introduce a strong element of composition in a frame. Recently, I read one reference that described the S-curve as "compositional gold". Some say that the S-curve is one of the only geometrical shapes that can stand alone as a main subject.
You never know when you'll come across a S-curve while shooting. Recently, I happened to notice one in an unlikely place--the interior of the Michigan State Capitol building. I think the S-curve significantly strengthened the composition of this photograph: