Not everyone wants and editor.
Not everyone needs an editor either. If you are working with audio recordings of oral histories the last thing you would be likely to want is an editor “correcting” colloquial phrases.
(Rest assured, I approach editing and transcribing very differently).
That said, there are good reasons why many writers would want an editor’s query, comments, or “correction.”
As a pre-teen, I was an avid reader of a YA series of books (I won’t disclose the title or author name here). I don’t know how many books I had read of this series when I was startled to discover–in the course of a few pages–that the protagonist’s hair color had inexplicably changed (that is, the author’s description of it had). Not only that, the actions of the character became suddenly inconsistent within a paragraph or two. Her shirt color may have changed also. My attention from the plot immediately was redirected to questions of the author and the publishing process–how did such oversight get through to print? I’m not sure I immediately stopped reading the series, but I did begin to question writing quality and suspect that many mass-market paperbacks were written fast to be sold quickly. This book was in need of an astute developmental editor and a line editor.
(No, I didn’t decide then and there that I wanted to be an editor, though this anecdote would have better cohesion if I had; my path to and from and English degree was meandering.)
An editor is essentially a mediator between author and reader.
Many authors, especially new authors, often don’t realize that there are various levels and “passes” to the editing process and tend to want to skip to proofreading when they really require additional substantive editing.
Editors tend to have their own interpretations and terminology preferences for the editing process. Basically, levels of editing include:
Developmental (or structural) editing: considers the logic of ideas, thought development, organization, and consistency.
Copyediting/Line editing: sentence-by-sentence grammar, syntax, voice, tone. At this level, there is much less emphasis on checking for accuracy of the information and additionally detailed analysis of writing mechanics.
Proofreading should not be brought into the conversation until the substantive editing passes are complete.