It reveals the blind spots our pitifully made eyeballs are flawed with... damn octopuses always make me jealous.
easier to see with this I think:
Close the right eye and look with the left eye at the white spot in the grid. The black cross on the left disappears (if your head is at the right distance). Or do it the other way around.
Usually the second eye can compensate. In cases like above, the brain will just make stuff up based on the surrounding.
It's a nice demonstration why you can't trust fully what you see (i.e. people seeing "ghosts" etc.) as you only get a modified, dumped down photoshop-version of the sensory input (which is already limited by itself).
More extreme cases are people that e.g. lost the whole right or left side of their vision. Their brain will just fill in the rest and then they get run over by a car because they though the left/right side of the street was clear but their brain just made that up because it couldn't see the car