This is also why you "see things from the corner of your eye." Your visual clarity is best in your central field of view, but your eyes are actually better at detecting motion at the periphery of your view. Your eyes detect the change in light better at the edges and your brain says "something just moved."

You can try this in your own house. Just find a place with almost no light, let your eyes get accustomed to the dark, and then try to look an objects straight-on. They're very difficult or impossible to see. Then try looking next to that object. You should be able to see it.