1. On this shot curve is more showing direction of tree, the way it has been bended by the snow.
2. Here the whole composition says for itself. The road turning left together with a sign and snow.
3. Here the curve is showing both the movement and the direction of the water current.
4. It seemed interesting to me, how curve puts such a static object as buildings into quite a dynamic composition.
Summarizing this exercise, I would say, that in most cases people use curves even without thinking, what they are actually adding to the composition. I guess that thinking more thoroughly in temrs of curves, diagonals and lines really improves the vision of the composition.