When writing a paper or article, you need to style and edit your writing in a way that will make it understandable to your audience. For example, you will need to present your ideas in a logical sequence, choose words that will bring across your ideas unambiguously, and watch out for words that have similar spelling but vastly different meanings (such as decreased vs. deceased). Using the services of an editor helps you make sure that you haven't missed anything, and takes the burdon off your shoulders if you only want to write down your ideas and have someone else take care of the language aspects.
However, it is importance to understand that there are several types of editing. These may be divided into two broad types: content editing (substantive copy editing / rewriting) and mechanical copy editing. In content editing, you want to make sure that your ideas are presented clearly. Imagine an upside-down pyramid: first, you need to edit your paper as a whole and develop your theme. This is called developmental editing. After that, in structural editing, you focus on the structure and chapters of your paper. Finally, you need line editing to make sure that your ideas are clear at the sentence level.
Now that your ideas come across clearly, it's time to focus on mechanical copy editing. A copy editor will read through your paper and make all the small corrections: syntax, grammar, word choice, punctuation, clarity issues, conformance with style rules (such as APA or journal-specific rules), spelling, etc. After that, a proofreader will scan your paper to catch any mechanical mistakes remaining and provide you with a final copy that you can submit.
An editor may provide several of these services or specialize in one of them. I usually provide copy editing and proofreading, in addition to editing of citations and reference lists, as well as styling and formatting, but may also help you with other issues that you wish to bring up. Whether you need to brush up your paper or only have it scanned for clarity and small mistakes, the purpose is the same: to make your paper shine.
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