Depth perception is important to primates.
All arboreal monkeys and apes need to be able to judge distance very precisely. For this reason we have two eyes in the front of our heads, offset by about 8cm. But even with one eye closed, there are a number of stimulus cues which contribute to depth perception. These include those listed above. Height in plane refers to the horizon being at eye level, and near objects varying heights above or below the horizon line according to their relative proximity to the viewer.