To add to my above post, color grading is another "artifact" that most film makers add. First you try to get perfect color for all your clips by color correction then you tweak the colors to your desired "look". Not many film makers color correct than stop there. Looking into a mirror and seeing perfection (myself not included) is not art. Just like there are different styles of oil painting, and some of the best paintings look nothing like reality, film makers are also free to tell their story not just through action and dialogue, but by framing, lighting, focus, coloring, and yes, adding "imperfections" to the images. A pristine image with perfect color might be a starting point, but as a final image for a work of art, in my eyes it would be pretty boring.