Most of us, even English majors, make grammatical mistakes. The difference between a copy-edited book and one that has not been copy-edited is enormous. Copy-editing doesn’t change the substance of what you’re writing about. In fact, it enhances it – clarifying meaning, correcting distracting mistakes.
A good copy editor will adjust your punctuation and spelling, question whether or not you really want to use jargon, make sure you’re using the right terminology, and keep you from embarrassing errors of usage. He will keep your language consistent from page to page, and ensure that you capitalize names properly.
Traditional publishing houses either have copy editors on staff, or outsource this function to qualified professionals. In both cases, every book that passes through a publishing house gets a thorough going-over to ensure that the manuscript is free of mistakes, and conveys the author’s message concisely.
Copy-editing is different from proofreading, which happens on a facsimile of the final product – while proofreading can catch errors of typography, copy-editing is focused on errors of meaning. Self-published books frequently suffer for want of copy-editing, and this is one of the differentiating factors that can make a self-published book appear less professionally-produced than a traditionally-published one.