I believe with painting, Value is the most important thing!
A value is a range from light to dark. Weather its refering to the paint being used or the combined result of my mediums once applied, dried and left alone.
We see what is lit and what is not we don't. In real life, the observable world works that way. On a 2 Dimensional surface this results in a pattern that can be one of the most important ways to clearly express in picture form.
So, by grouping my values I get a firm handle on this pattern, a strong visual image appears, by simplifying to even 2 or 3 ranges; dark, middle and lightest tones, this will create the clearness of intent I need.
And make better paintings !!