In many cultures, the eyes are regarded as the doorway/path to your soul. Therefore, in many ways, eyes are often manipulated to show the will/emotions/drive of the creature in question (and by creature, I include animals, aliens, humans, anime-pseudo-beings and all other lifeforms that are shown with eyes).
In contrast, what does not have eyes, we often perceive as dead. Even creatures that are presented without eyes, often carry a bandage og other device covering the area where the eyes would normally be shown.
To give life and passion to a character in a show, they have to be designed with eyes or other seemingly visual-oriented details.
Also notice: Often bigger eyes are pictured as more passionate, while small/tiny eyes are often used to convey negative traits.