Does our brain contain an innate function about closing only one eye?
I'm only a 17 years old student but i guess this is the answer(its just what i guess, haven't seen it in any books or research):


This is a picture from the book Life.
When both eyes are working they send signals to brain.
signals from things on the left side are sent to right side of brain and signals from things on the right side are sent to left side of brain(coming from both eyes).
as you can see both eyes participate in sending signals to right and left hemisphere. 

On the right side of picture you can see visual cortex of right hemisphere , it has signals coming from both left and right and makes the thing we see.

While seeing through only one eye (or when you cut optic nerve from one eye) there is still signals coming to visual cortex from the other eye and that makes image of what you see.(when you close one eye there is still little signal from it if you are facing a bright light source, but signal from the other eye has more effect as its more powerful( try it by closing one of your eyes and put a torch in front of only the closed eye, if the torch is bright enough you can see bright thing there))

when you close both eyes the signals coming from both eyes are almost the same and if you face a bright thing, you almost see it.

i just guess that's the mechanism :)