Journal copy-editors: why is their work productive? You worked on your manuscript and its language for a year and a half, carefully choosing every word; what is there left to do for the journal's copyeditors after acceptance? What is the point of copyediting your manuscript?
The writer of a document is often the worst person to verify its wording. There are two reasons for this. 

Often, when we write something we have a certain mind set. But we aren't perfect, so we occasionally make mistakes. Then, when we revisit it, we make the same mistake again, "seeing" on the page what we thought we wrote, rather than what we actually wrote. 

Second, and more important, the author of a manuscript has a lot of background knowledge that they bring to bear on the subject that isn't actually written in the document. The reader, on the other hand, may not share this background, and usually won't to the same degree. 

An informed copyeditor can correct for both of these situations. 

I've learned that I can't really reliably proof my own writing and make both of the above "errors". 

But copy editors don't just make changes without the advice of the original author. They provide a new version and the author gets to approve, reject, or improve the "corrections". It is a very valuable service. 

But even just in the use of language. A good editor can improve the presentation of ideas by improving the structure of complex sentences and suggesting where the statements made could be confusing to a reader. 

It does require some subject level knowledge to do it well, of course.