Copy editors provide a safety net for a publication, catching most of the problematic stuff dropped from above. They are a curious breed: trivia experts, steeped in popular culture (helpful for pun headlines, none of which Google gets), usually voracious readers, often unappreciated.
A copy editor’s work is largely invisible, until she misses something, in which case she takes the blame. But most important is that a copy editor stands in for the reader, gingerly reshaping, clarifying and correcting things before the reader can see them and post an excoriating comment.