In the era of self-publishing, it’s tempting to cut corners. There are how-to guides available online for virtually every aspect of production. While there certainly are parts of the process the author can take on (i.e. formatting), editing should never be one of them.
The reason boils down to objectivity. After spending months fleshing out a rough draft–and probably going over it multiple times–there is no way for an author to remain truly objective about his own work. Consequently, there are things far more important that spelling and grammatical errors that can slip under the radar because the plot is too clear in the writer’s own head.
An author can go over his work a dozen times and still miss plot holes because his mind is filling in the gaps. Necessary bits of description can also go missing due to the writer forgetting the readers do not have the same rock-solid mental images he does while he’s reading the text (which can make something/someone that could have been impressive overly vague or bland).
Additionally, there are certain quirks a writer may have and not even realize. Overuse of particular words is a big one (“as” is very commonly abused), as are ignoring contractions in dialogue (which can make characters sound robotic), and improper use of certain punctuation marks (semicolons and hyphens are frequent offenders).
Telling the story is the writer’s job, pure and simple. However, no author is fit to polish his own work alone. Because after staring at the same spots for so long and trying to make them shine, he often fails to notice the rust hiding just out of sight.