A book doctor (as opposed to a copyeditor or proofreader) will read your book, looking for big-picture issues that need addressing, such as development, structure or organization, and flow. (When reading novels, they look at plot, character, pacing, and other elements vital to lively and salable fiction.)
An editor you pay will be more objective than a teacher, writing group member, friend, or spouse. They can help you fix what’s wrong with your novel, though they cannot guarantee publication. They also can’t turn bad writing or a clichéd story into a best-seller.
Check the track record of the editor you wish to hire. He should have a background in the particular field of your manuscript (novels, plays, etc.) and should, if asked, be able to provide a sample of a former critique to give you an idea of the nature, extent, and content of the criticism provided. Usually, reputable book doctors or editors don’t take on projects that they feel have no chance at traditional publication (if that’s your goal).
Whether or not you should hire an editor is totally up to you, but most manuscripts do benefit from at least a line edit (or proofread) before submission. If an editor or agent has two manuscripts on his desk, and one needs a heavy edit, and the other looks polished and ready to go, it’s not hard to know which one he’ll prefer and be more likely to accept.