Style manuals are overrated by editors and underappreciated by noneditors. Editors too often adhere to a style manual's rules without really thinking about the purpose of a style manual; noneditors dismiss the manuals because of their perceived rigidity.
Style manuals generally are relevant primarily for nonfiction. They serve the purpose of ensuring readers that certain conventions are followed so that a reader can travel from book A to book B and not be diverted by personal preference differences. More importantly, they ensure that bibliographic material is complete and familiar and usable, something that is particularly important in nonfiction. It should be noted that nearly all style manuals focus on scholarly work and need to be viewed from that perspective.
The primary nonscholarly focused style manuals, like the Associated Press and the New York Times manuals, are focused on a specific industry and are designed to bring a sense of uniformity that readers expect to the particular product.
I discussed some of this in my article
Editors & “Professional” Resources: A Questionable Reliance.
Although there are a lot of authors who echo Moejoe's loathing of editors for various reason, there actually are some excellent reasons why a professional editor is important. Of course, if you think like Moejoe and price your writing as free, then it makes little sense to invest in professional help. But if you want to switch from a day job to full-time writing as your sole source of income, the balance shifts, I think.
For whatever it is worth, I have also written articles on about professional editing, what editors do, what to expect, and why authors need professional editors. I invite you to peruse them at my blog An American Editor (link below in signature). Search for Professional Editors or use the Filed Under tag Professional Editors to view them.
One last thing: As another poster mentioned, the biggest problem when self-editing is seeing what is really on the page rather than what you expect to see. I wrote about this in
The WYSIWYG Conundrum: The Solid Cloud. You might also find this worthwhile reading. Although this is a particular problem for self-editing, editors, too, suffer from this problem and often see what they expect, which is why few books (admittedly, I've never encountered one) are error-free.