A copy editor should be one of the many pairs of eyes that review your manuscript before submitting it for publication. I have read approximately 250 books since I received a Kindle for Christmas 2012. In 90% of these books, I have found multiple errors in grammar, spelling, punctuation and consistency. For example, in Chapter One of one book (that shall remain unnamed), the heroine was described as being seventeen years old. By the end of the book, in a span of approximately two weeks, she was nineteen. This kind of simple mistake can be extremely annoying to your reader! It can also distract from the plot and the overall message!
Even if you only ask a friend to read over the manuscript, not all errors will be found. As an author, you must engage the services of a proofreader or copy editor to perfect your writing. Not all mistakes will be caught by Spell Check or Grammar Check!