There are at least two good reasons.

When you’re talking to someone, you have opportunities to correct any misunderstandings. If the other person looks blank, you can explain — or they can just interrupt you with a question. In writing, however, you have one chance to make your point, so it’s important to get it right the first time.

Also, you’re competing for readers’ attention. If you make life hard for your readers — through misleading word choice, errors in spelling or grammar, or sloppy logic — they are likely to respond by doing something other than reading and properly appreciating what you’ve written. After all, something that might be more interesting, or more fun, is rarely more than a mouse-click away.

A skilled editor can help you make your writing clear and easy to follow — a pleasure to read. Don’t give your readers (teacher, publisher, customer) any excuse to stop.

Writing without an editor is like flying without a copilot, easy to get lost or make a mistake.