What is wind doing in this chapter? You can't see it. How do you composite it into your shot? Although it's invisible, re-creating the effects of wind on your shot can help sell its realism. Besides the effect of wind on particles, what other roles does wind play?
As I write this chapter, it is a warm, still, sunny day, but in my backyard leaves and tree branches do sway gently from time to time. If I wanted to re-create the scene from a still or 3D render or if I wanted to add an element to it, the result just wouldn't look right without these subtle bits of motion.