First, a professional editor has developed his or her craft over time. He or she has learned different techniques for how to identify errors and correct them, met with successes and failures, and accumulated a set of experiences that have made him or her a better editor. When a professional editor looks at your work, he or she can help find errors you may not have otherwise noticed or even considered.
Secondly, a professional editor can approach your writing from an objective perspective. He or she can help transform your writing by focusing on issues such as the logical structure of the work. There is also the overall quality of your writing and whether your ideas are coming through clearly. Professional editors are not influenced by having written the work themselves and are therefore able to give you honest, pertinent feedback.
Thirdly, a professional editor is skilled at looking at the tedious details to identify as many errors as possible. Examples of these are not limited to subtle differences in word choice and correct comma placement, but also include the logic and flow of the writing, as well as consistency in your voice, style, themes and details.
Finally, a professional editor offers you the chance to fix any small or large errors before the people who matter see your work. This can certainly save you potential grief, embarrassment or failure, but—more significantly—it can also give you the greatest chance of success. While it may be difficult to receive professional criticism from your editor, it is better to receive this criticism from a trusted professional before it matters than to receive a poor grade or rejection after you submit your work.