A useful article from Smashing Magazine.
According to Wucius Wong in his book Principles of Form and Design (page 42), point, line and plane can be considered conceptual design elements because, although they are not always explicit or visible, they seem to be present by implication. He explains how an angle, for example, implies the existence of a point and how lines, by marking the contour of an object, imply the presence of a plane.
In most art and design classes, students are asked to analyze the structure of a painting or design in order to better understand principles of organization. These linear studies usually have no relevance to the student outside of the class. But these exercises hold an important lesson, which is about learning to abstract images — and even our surroundings — into linear structures in order to learn about layout organization.