That is one of the main ways video codecs shrink file size.
People notice visually mainly what moves in a visual framework.
Once they've taken in the background once, the foreground is everything, where the action is. Only when something moves in the background is it noticed, and not always then, like the gorilla. That more than halves the size of your signal once again. The viewer gets enough information for the brain to process to keep up a modicum of reality, in a similar way to how the brain works. We don't process every single signal that comes out of our rods and cones fresh every instant, because our brains also have limited processing power.