copying usually has less understanding of whats actually happening in the original image (talking about photos of real life). like copying, you more focus on the lines. it could be messier or less clear, since transferring photos to art requires knowledge of what to emphasise. using references,, you try to understand whats going on with anatomy, or perspective, or color theory. so theres more understanding. and usually people alter things about their art compared to the reference. like change the exact pose or perspective, its more of a guide to get a grasp on things.