I have recently found an eloquent description of why writers need editors.

"You hold the whole text in your mind, and you have developed its ideas in sequence right to its conclusion. You can't now put yourself in the reader's place by somehow 'unknowing' any of this. A copy-editor, though, will bring fresh eyes to your text, helping you to reveal your concepts in a logical order."

This comes from the Society for Editors and Proofreaders. Written on behalf of a professional organisation, it illustrates well that a good editor is the best go-between 'twixt a writer and the reader.