The eye that appreciates drawings to their fullest extent, comes from past experiences of hunting for forms in nature in order to translate flat vision into meaningful forms on paper.
It also comes from correct knowledge of anatomy and structure of the human body.
With the developed eye, one can marvel at the succesful drawings and see mistakes of some drawings. It enables one to become inspired more easily by other artists' works.
The eye comes from more direct contact with what really exists in nature. In a sense, this is an example how 'foundation' built upon life drawings can help one further his imaginary drawings.