The same thing applies to the depth. Typically a camera would be focused on a certain part of a scene which the user adjusts by controlling the focus ring. In this way you may have the object in the foreground in focus, while you loose out on the detail on the background, which is blurred, or you can also adjust the focus ring so that the background comes into focus, while the foreground goes out of focus, is blurred.
Just as we wanted to get the entire brightness range in view without loosing detail, so do we want the entire depth range to be in focus, just as we want the entire breadth and height to be visible. So these are things that we want the camera to be sort of be performing uniformly well across the range in all three senses.