You might craft the greatest story, have the most incredible prose, and have a strong social media following. You might be the next J.K. Rowling, Stephen King, or Neil Gaiman. Tragically, the world won't know it if they are distracted by errors.
Imagine if Rowling had accidentally published a line with Hairy Potter in it, or if Carrie had telepathic powders instead of powers. Books are meant to immerse you in another world or in another life. The best way to break that immersion is to let typos slip through the cracks.
Editors don't just clean up syntax, spelling, and grammar. Developmental editing is a huge part of an editor's job. Developmental editing looks at the substance of the story, checking for inconsistencies and big picture problems. Did you change a character's name halfway through writing? We will catch that. Have you established rules of magic at the beginning of the book and made sure that every single charm, spell, and hex has followed those rules? Editors will.
But editors aren't just for authors.
Small businesses are even more at the whim of consumers. We all know how important good content is for businesses, but that content is only as good as its substance. Editors are crucial for small businesses to attract and retain consumers with their content. Why spend hundreds of dollars to have blogs and posts written for you without having the professional eye of an editor making sure that your copy is the best that it can be?
So, in short: Yes, you need an editor.