There is no great writing, only great rewriting.
- Louis D. Brandeis
Good writing is more than spelling and grammar. It is considering your audience, hooking the reader, writing tightly and actively, being specific, and placing content in the correct place. Hired writers and editors work for you. They help you to convey your message in the best way possible.
Editing by a qualified editor helps to remove ambiguities caused by improper comma placement (or missing commas), subject-verb agreement, word order, and more.
Well written and edited material speaks of care and attention to detail. The whole document is your signature.
See Five Good Reasons to Hire an Editor on the Editors' Association of Canada website.