Humans have two eyes that see the image at slightly different angles and for perception depth it is important to see the object from both the eyes. Retinal disparity is the way which helps in depth perception.
In this situation, the image seen by both the eyes at different angles is processed by the brain and after processing it is seen as a single image with the correct depth of the image. In many situations retinal disparity comes into play like diving.
Therefore if a person lacks an eye then retinal disparity is the visual cue that would be lacked by him.
