Mood or atmosphere is the icing on the rendering cake. When used with thought, it can completely transform a scene and can inject a sense of power and drama at the same time. You can use atmosphere to enhance a "normal" rendering into one conveying power and beauty of nature's finest moods. We can use it to lose unwanted detail, or suggest mystery, impending doom, a light, airy afternoon, tranquil moments, and much more.
Mood can be especially effective in highlighting a center of interest or suggesting a narrative. Try to get out of the habit of always rendering fields green, clouds white, or mud brown - flood the whole composition with a color mood to suit your idea, even if it conflicts with color we normally assume to be correct. Those who fail to see pink mud are truly restricting their imaginative palette.