There are a variety of simple reasons any person could use an editor.  I could probably link you to twenty articles from talented, knowledgeable people who could explain (using science) about how your brain fills in missing words because you know what’s supposed to be on the page.  Your brain filling in gaps is the same reason reading a document backwards doesn’t always allow you to catch mistakes, and it’s the same reason that causes you to turn in an assignment with bad subject-verb agreement. Your brain knows it should be there, so your brain puts it there.

But forget the links, and forget the science (for now). You know why you need an editor? Because I need an editor. And I work as an editor.

I once spelled “grammar” with an “e.” Seriously. In fact, I did it more than once.

I have been trained–really and truly trained–not to do these things. And I do them anyway. Because my brain is your brain, and an “e,” when you expect an “a” will fool your brain.