In some scenes the narrative content - the "what it is a picture of"- comes from the darks, which are usually applied late in the sequence of layers.
Los Tamales Mejores Tom Hoffmann
Imagine this painting without the darks. There would be
very little to tell you what the lights and middle values are describing.
Sometimes, though, most of the story is told before the darks go down.
Soledad Tom Hoffmann
Even without the few strong darks the illusion of substance and light would be clear. Although the lights comprise most of the image, it is the middle value layer that carries most of that information.
For homework, look for an image that you suspect may owe its content to the mid-value shapes. Make a study in which the lights are left white, and everything else, including all the darks, is painted in a single mid-value color. Bring both the study and the photo to class, if possible.
Here are a couple that might work, if you don't find one of your own.