Professional editing is never just a check of spelling, grammar, and punctuation; neither is it to read through a story to see if it has merit. It is a skilled job of presenting an author's words, intentions, and meanings to the reader; in a clear, concise, and coherent manner. With this in mind, the journey to publication is a long-term project that requires commitment from both editor and author. It is not a quick fix!
In traditional publishing, a manuscript undergoes three levels of editing before becoming a published book. The first stage, structural editing (also known as developmental editing) is usually carried out by a comissioning editor. The second stage, copy-editing, is carried out by a copy-editor. The third stage, proofreading, is carried out by a proofreader. Each stage comes with its own skill set and the editor concerned will be trained in that discipline.
It really isn't possible for a writer to edit their work as a professional editor would. They lack the necessary skills, don't have the required objective viewpoint, and are often unaware of the pitfalls that occur during the writing process.
Self-publishing without proper editing will ultimately have a negative impact on an author's reputation; therefore, professional editing is a must. It ought to be considered as an investment for their future as a successful independent author.