1) Messages get garbled as they travel from brain to page. What sounds logical and coherent in your brain often comes out garbled on the page, but you may not notice. An editor will pick up on this and help you express it clearly.
2) To maintain credibility. Every reader gives an author an amount of credit when they start to read. However, this can be quickly exhausted if a document has typos, the grammar is wrong or the material is poorly written. Conversely, copy that is cleanly written and error free gives readers confidence in its quality.
3) What appeals to you may not appeal to anyone else. Our ideas are borne out of our own unique view of the world. However, we are all subject to a myriad of cognitive biases, blindspots and weaknesses that we may not even be aware of, but will weaken our message. An editor has a different point of view, and thus acts as an invaluable and safe means to road-test these ideas before publication.
4) Killing our darlings is hard. We get attached to pieces of prose we have written that we think are wonderful but are superfluous. An editor can help with the hard work of identifying and removing them.
5) We all make mistakes. Deadlines, phone calls, emails, meetings to go to – life is pressured, and mistakes are inevitable (see picture). Text cut and pasted in wrongly, tables put in twice, sentences not finished – all these can slip through without an editor to check.