By only seeing one rectangular side of the cube, you can't say anything about its real form. Rectangles can be a side of a cube or the basement of a pyramid or prism. A view of three sides gives a clearer presentation. But even in that case each side of the cube is visually accepted in a deformed way. Parallel lines become unparallel, shapes and forms change.
Perspective is a science that studies optical changes of forms, sizes and objects in nature.
Perspective is also a way to reproduce 3D images in two dimensions. Following perspective rules is a necessary requirement to draw realistic forms of the surrounding world. When you look in one direction, you see just a part of the surrounding world. Light rays, reflected from a visible object are pointed to an eye and focus in pupil. So-called view rays are directed from your eye to the object.