I hear this question a lot. Over the years, I have changed my mind from not wanting a professional editor to not wanting to publish something without a professional editors review. My change came about from reading a lot of books and homework that were not professionally edited and seeing how many of them had mistakes in them.
I asked the authors of those books if they had paid an editor. Each one said no. Their reason was that they didn’t think they needed it. I then explained that the errors in their work made their book less professional. I have also read a lot of work submitted by college students who are not allowed to hire an editor, but achieve the same result. Their work had a lot of errors that took away from their grade. My point is we all make mistake in writing.
To test my theory I sent some of my old work through an editor. She found a lot of mistakes. In some cases, I had read the work a half dozen times. What I didn’t realize was that I was reading the work as it was in my mind. That included mistakes. I was reading right past them.
Now I pay an editor for all of my prose. I am continuing to go through my old work until I get it all professionally edited. Why? Because I want to be proud of the work I do. I cannot in good conscious be proud of something that might contain errors. To answer the title question, I use a professional editor to make my work the best it can be.