For those who have a relatively strict regime in self-editing, regardless of your method, a professional editor is someone looking for things that slip between the cracks and, to be realistic, it's often arguable whether our contribution is a viable or economical choice. Sometimes you can be editing a manuscript and thinking, "Wow, you just needed a proofreader" (or something along those lines) and then you discover a BIG plot hole or issue that makes your editing all worthwhile.
But I was referring mostly to writers whose strengths lie in storytelling and premise, but their technical prowess in writing keeps breaking rules — and often they introduce new errors in their attempts to fix perceived problems in the existing MS.
I'm suggesting that many writers who can't, or don't, self-edit in detail shouldn't stress too much about the nitty-gritty of their manuscript when their intention all along is to commission a good editor. We don't charge by "the misplaced comma".
I'm currently at the bottom of the world where the internet is powered by hamster wheels, so apologies if I don't reply to my own thread too much.