As an author you want to connect with your audience, whether you have written a doctoral dissertation, technical report, journal article, memoir, or fantasy novel. An editor can help enhance user experience by removing distractions such as grammar and spelling inconsistencies so that your readers can focus on your message and hear your voice come through loud and clear.
Spelling and grammar software, including the ones based in artificial intelligence (AI), are a great help -- I use them, too -- but they can't read and understand a text in the same way a human can. Editors can bridge the space between your ideas and your readers' understanding.