For instance, you want to have an scene where a ship is flying into the sky and you want it to disapear within the sky in the distance between the clouds. Chances are this will get ruined by clouds covering the ship, so you have to wait for the clouds to be at the right position so the ship goes throght a gap in the clouds. Needless to say this is insane. We should have control over the position of the clouds with the background keyframe.