In all my 2-D art classes in college (and maybe also in high school -- I can't remember that far back!) teachers repeated the same mantra: "Pay attention to your values. Do value studies. You need more value change."
I do recall thinking 'That doesn't apply to me; I'm a COLORIST!' How wrong I was, and how clear it is today. Color gets the credit, value does the work.
Human brains, indeed mammal brains, use value change to identify movement and other kinds of information that might be crucial to survival. So if we want our paintings to be relevant to viewers, we need to use value change.