Why does an author need an editor?

Question

I'm firmly in the camp of "your novel needs an editor, and that editor better not be yourself." But recently I was asked why that was, and I couldn't articulate an answer that was passable. Young novelists often underestimate their need for an editor, so frame your answer in such a way as to convince one of them.

Answer 1

Because a second pair of eyes will always find mistakes the author missed - in writing, mistakes are like cockroaches: where there is one, there is a thousand.