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.