In the bonus documentaries that come with Star Wars: The Phantom Menace, there's a long demo of a waterfall scene that didn't make the final cut because it slowed down the action. Most of the components of the scene are digitally created, including all the water effects. The artist shows how he created the waterfall from image samples and overlays -- and then added noise and film grain. Film grain. Because the image looked too crisp and perfect.
So by making a digital image look like a photograph, it becomes more "realistic" to our eyes. Isn't that interesting? Photography connotes realism. And we make this judgment -- realistic or not realistic -- when viewing an image, even though we should consciously know that photos can be staged or completely manufactured.
"The medium is the message." -- Marshall McLuhan. When a digital image simulates a photo, is that two messages? Or is it just a mixed message? ;-)