Depth perception. You need two eyes in order to have true depth perception. Interestingly, people who have had only one functional eye since birth or early childhood can judge distances about as well as two-eyed individuals (by comparing shadows and sizes of objects). Those who lose the function of one eye later in life may have some difficulty judging distances, but eventually can manage most tasks normally.
Binocularity. To have depth perception, you need two eyes that work together to make one picture.