Effective copy editing removes verbal noise from your manuscript. Great copy editing is invisible and makes you look like a better author. I worked with one of our authors helping to publish his first book, a spy novel based on his own experiences. I said I could improve his story by cutting 15-20% of the words in the 5,000-word sample he gave me to review. He was skeptical. Later, when he showed the before and after drafts to several friends, the unanimous verdict favored the tighter version. Here's a good article for self-publishers on the promise and pitfalls of doing your own copy editing. Summary? Proceed with caution. Get a good editing job done on your manuscript.
And, yes, we know we've misspelled copy editing in the title.