Why would anyone paint a rock? One great reason is called a 'study'. The idea here is to add depth and richness to your mental library by learning the details of real-world stuff. In this case I'll be demonstrating a study of slate, though the same concepts apply to all subject-matter. If you want your illustrations and concept art to improve, studies are a great way to do it.
If you're unfamiliar with visual research or studies, make sure to check out the videos linked to the right.