Everyone needs one.
Even if you're self-publishing. Actually, especially if you're self-publishing, because when selfpublishing you're in control of all aspects of the production and it shows that you don't respect yourself or your work if you don't get an editor.
I don't mean a friend who always got good grades in English and is willing to look it over, either. I mean an editor. At the very least a copy-editor who will go through and read each word and make sure the commas are appropriately placed and that you didn't change a character's nickname from Timmie to Timmy halfway into Chapter Five and that you're using the correct spelling of 'disc' for the context and the correct spelling of 'grey' for your country.
Editors tend to be an investment, and sometimes selfpublishers will balk at the price. But when you consider how much of your money you've already invested into your work in terms of hours of focus, it's usually going to be worth it.