So when I move around to get the right angle and compose my shot before shooting, isn't that visualization (or previsualization a la White)? So who doens;t do that?
In real life, the scene looked like this, but not identical to this - the drama was there, but muted:
However, while I stood there on the foot bridge looking at the water, the trees, the brightly lit clouds, the dark foreboding clouds, the mountains in the background, the reflections in the foreground, and in particular the hydro poles and lines, I
visualized the print I could make, if I exposed the (35mm) film carefully, and printed it interestingly.
The Zone System is all about being able to, in your mind and with the photographic tools at your disposal, envision what the final results can end up to be if you make some of the photographic choices available to you - sort of like having Photoshop adjustment sliders in your head, and a viewing screen in your mind (to stretch an analogy a bit).
The System part is merely the part that attempts to make the result predictable, when you choose certain exposure, film development and eventually printing options.
FWIW, once I have a darkroom work print I like, I go another step down the visualization road - I envision how toning might enhance the result. Sometimes that mandates a slight change in the printing as well. In the case of this image, different toning choices and different printing choices result in subtly different results - which as much as possible you want to be able to visualize before you make the print. Here (from Postcard Exchange number 36) is the result of some of those changes: