The point is that an editor whom you're paying, who's not part of a publisher's stable of editors — freelance or otherwise — instantly has a vested interest in making you, the writer, happy. That usually means the editor is less likely to be completely impartial about your work — and your mistakes. E-publishing authors listed having a good editor as the most important facet of their writing.
Whether you opt to self-publish, e-publish or traditionally publish your work, the same principle applies — a good editor is worth her weight in gold.
Happy writing — and editing!
Resources: Preditors and Editors and Manuscript Editing