One fundamental requirement for any visual system (biological or artificial) is to obtain the depth information of an object in addition to its position in the visual field, so that the 3D information of the environment can be deduced from the 2D image plane (the retina, the camera film, or some CCD device).
Depth information is crucial to many organisms. For example, predators have to accurately judege the 3D position of potential prey (eagle vs. mouse, cat vs. bird, frog vs. insect, etc.). Moreover, depth informaiton is important for recognizing shapes of 3D objects (e.g., disk vs. sphere) and to distinguish figure from background.
Depth information can be obtained by the visual system in different ways (monocular and binocular) and at different levels, from primary visual cortex where basic visual information such as orientation and spatil frequency is first detected, to some higher perceptural levels where more specific knowledge about the objects (such as its size, shape, texture) is available.