Different strokes. All the movie shoots I worked on they used them for what I mentioned. Didn't say anything about exposure, but light placement. Also used in tight location shots where the director has a hard time being in the same room.
Apparently Jerry Lewis was the first to use it to direct himself and get instant feedback.
Actor-director Jerry Lewis invents and makes first use of a video assist system on The Bellboy,
an idea that Francis Ford Coppola perfects on One From the Heart (1982).