When I'm getting ready to paint a larger painting I sometimes get too anxious to get started and forgo doing a series of color sketches to explore other possibilities in lighting and color and I end up regretting it. Even if I have a sketch that I've done outside I still try to find different ways of approaching the color or composition. The habit of doing 3 to 5 small color sketches (3 x 5) or (4 x 6) is a good way to develop your sense of color and composition. Just trying to see your subject different keeps your mind working instead of copying the photo reference which can cause the painting to go stale.
Changing things in your reference forces you to think about the effects you're trying to create and what you need to do to achieve them. It also helps you develop a better sense of color. I'll do 2 or 3 of these small sketches on a strip of canvas or canvas board, about 3 x 5 inches. I don't spend more than 10 or 15 minutes on each one, just enough information to push the overall color scheme or values or move the planes of landscape around to explore a different composition.