I think every writer needs an editor.
Each writer may have that group of individuals that help with early reviews of the book to provide feedback on the plot, the characters, and the style.
But, in the end, i believe there needs to be a single individual that you can depend on to review your work with professional detachment and provide the feedback necessary.
For the self-published, there are independent editors. They aren’t cheap—the average story/copy editor, working in the industry, appears to charge around $.03/word, but often gives you two rounds of editing for the price.
I know that I worked so long and hard on this first book that after a while I couldn’t see the book for the words. There was no way I could be judge the results of my efforts.