I have to add that for a large fresco sinopia is essential as a composition reference, as the painter has to work in sections attaching/lining up giornatas. Without to scale reference it would be quite a difficult if not impossible task.

Just imagine that you are to paint a complicated composition on 36" by 48" canvas, but the task is to paint it in 10"X10" odd shaped segments starting from a blank canvas. You are allowed to sketch and paint only one little segment at the time and go to the next one only after the previous is completely rendered in color. You are also not allowed to paint over previously painted segments.