One of the most difficult tasks for a writer is to edit their own work. It’s nigh on impossible to retain objectivity and it becomes extremely difficult to pick up any editorial mistakes. The writer is so absorbed to what they are trying to put down on paper (or keyboard!) that the eye will slide right on over an error. That’s what editors are for. We are the fresh pair of eyeballs.
Just like a car mechanic will service your vehicle, give some sound advice on how to take care of it so an editor works the same magic on your written word.