An editor is a fresh eye to your document: It takes a fresh eye to see the things you yourself may have missed after several hours and hours of drafting and re-writing the same document.
An editor saves you time: Writing, editing and re-writing can be very time consuming for the average reader, especially one seeking a perfect end product. This is where an editor comes in. What could take you hours, weeks, or months to edit can very easily be edited in half or a quarter of the time.
An editor is a fresh mind: If you've ever spent time trying to edit your own document, you can definitely attest to the fact that the mind gets tired after a certain number of reads and attempts at editing. This is when an editor can save you time by bringing a fresh eye and mind to your document, allowing you to focus on the things that matter most to you.
An editor is a rational voice: Because an editor is not emotionally attached to your project, they are better able to bring a rational and unbiased voice to it from a reader's standpoint. A good editor should not be afraid to tell you the truth and help you bring out the best in your work.
So, before you submit your next piece of writing, be sure that you are presenting your best by having an editor look it over for you.