Good art/design is based on the interaction of light and dark; we call it Value. Seeing the Value pattern is key to strong work. That's why painters squint: at the still life, at the landscape - squinting helps to better see the Value pattern in the subject. Seeing the light and dark pattern and translating that to a 2 value notan sketch creates a Value foundation for the final work.
We have all seen work where the artist has no sense of a value pattern; it appears spiritless and blah.
In a great painting we may enjoy a stunning image or beautiful technique but you can be sure - behind every GREAT painting is a strong Value pattern, planned at the sketch/notan stage to create a well organized arrangement of light and dark.
Above is a simple class setup and the notan created for the painting. Can you see the notan in the photo of the pears?
French Impressionists used the notan idea as did the great illustrators of the 19 & 20 century- I am thinking of Howard Pyle and Norman Rockwell.
Call up some their work on the internet and take a look. Squint and see if you can see the notan - or the strong pattern of light and dark.