Have you ever come back to your work after several days and realized just how many things are wrong with it? An editor has the same effect. They see things you do not. In truth, another set of eyes always does. But editors are professionals. They are the difference between you working on your own car and paying a mechanic. Of course, the former is cheaper, but the latter is often needed.
You will spot certain mistakes. They will spot others. A professional editor is paid for their grasp of language. They’re an expert. Use them!