When viewing with two eyes, the muscles that control our eyes work automatically to focus both eyes on the same spot. Each eye receives light photons that are focused upon the retina by the lens to form an image. The 'image' is transmitted by the optical nerves of both eyes to layers of brain tissues at the back of the head. The brain then processes both the images to form single 3D image. Basically, if you close one eye, you can still see, but in a single plane. So even though your brain tries to add depth, what we see is a flat a flat image with one eye. But with two eyes next to each other, you can you can view the relative depth.
Because the eyes are a few inches apart the images are slightly different permitting depth perception. The left and right eyes balance the image’s depths and shadows. This is the reason why we see the image shapes. Both eyes together help to see the distance of the object.