The Perspective viewing projection has a Center of Projection (“eye”) at a finite distance from the projection plane (PP) So the distance of a line from the projection plane determines its size on the projection plane, i.e. the farther the line is from the projection plane, the smaller its image on the projection plane.
Perspective projections are used to produce images which look natural. When we view scenes in everyday life far away items appear small relative to nearer items. A side effect of perspective projection is that parallel lines appear to converge on a vanishing point.
An important feature of perspective projections is that it preserves straight lines, this allows us to project only the end-points of 3D lines and then draw a 2D line between the projected endpoints.