Here is a painting I finished earlier in the week. You can see if I desaturate the color and turn it into a black and white image that the areas in the background are much lighter. We know they are distant because we have the impression of atmosphere being present between the girl and the background. This one is a bit exaggerated to make a point but paying close attention to the lightness or darkness of the subjects in your painting can add interest and variety. If everything was painted equally dark in value the painting would have less impact and the viewer might feel compelled to look more deeply into the background. It's a portrait so the focus should be on the person and this is one way to achieve that.
Here is a painting one of my students did a few years ago. There is a big value change between the sky and the objects on the ground but he could have made the trees in the background lighter and he could have used lighter tones on the barn that is further away to add more depth to the painting. I'm not sure what the white thing in front of the weeds is (maybe a guardrail?), but it provides a perfect example of how the VALUE of a subject can really draw the eye. Even when you don't want that to happen. I suggested he change that part but he was kind of a stubborn guy. The nice color scheme in the painting helps to overcome the lack of values, but it could have been even better.