On the standard photography, the camera can not bring all the subject into focus even with the narrowest lens openings when the subject is at the oblique position to optical axis of the lens. (fig. left)
For the cases like this, tilt is the best way to bring all the subeject into focus even with the larger lens openings. This is, according to Scheimpflug Rule (fig. right), a way of adjusting focal plane by controlling optical axis of the lens, and one of the basic operations of camera movements.