To get clear 3D presentation of an object's forms you should have a very specific point of view so you can see the real volume of an object in all three dimensions. 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.