You want to keep "verticals," as in walls, straight - but usually you'd rather show more of the floor than the ceiling. Being able to shift the lens up or down

You want to keep "verticals," as in walls, straight - but usually you'd rather show more of the floor than the ceiling. Being able to shift the lens up or down while keeping it level lets you do this. You can just tilt the camera and fix the verticals in post, but then your composition will be thrown off as some of the image will need to be cropped.