Professional editing is never just a check of spelling, punctuation, and grammar. It is a skilled job of preparing a writer's manuscript for publication; to ensure it is clear, concise, and consistent to the reader.
In traditional publishing houses, a manuscript undergoes three levels of editing before it becomes 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 is trained in that discipline to look for specific issues.
It is not possible for the writer to edit their own work as a professional editor does. They lack the necessary skills, they do not have the required objective viewpoint, and are often unaware of the pitfalls that can (and often) occur during the writing process.
Self-publishing a book without proper editing will ultimately have a negative impact on an author's reputation; therefore, professional editing is a must for any writer who wishes to self-publish. It ought to be considered as investment in their future as a successful independent author.