Size helps a person determine how far the depth is. For example, when you look at Borgata coming down the white horse pike, you know your no where near close. The building looks smaller then it does when your standing next to it. Depth plays an important part in our daily lives. From driving, putting food into our mouths, or simple as grabbing objects. If one was to lose depth, one would really see a 2D world. And if someone only sees a 2D world, they can't do anything without running into objects, or doing normal daily task like driving.
I find it interesting that depth is that important in human functioning. Ill leave you with this, if you picture our world as we see it as, 2D, humans can't drive or grab objects properly. So what could we do if we can't grasp objects because of seeing 2D?