We know, because we have looked at the internet, that few people can write effectively, and we also know that all human beings are prone to error. Engaging an editor compensates for this state of affairs.
First of all, your editor will catch lapses in spelling, punctuation, grammar, and English usage. Trivial details as they are, they give readers an opportunity to discount your competence and dismiss your message. Your editor works to make your text clean.
Then, do you actually know what you are trying to say? When we write, we have an idea in our heads of the meaning and importance of what we are attempting to say, but what appears on the page may not correspond. Your editor will keep asking what you mean here and whether you meant to say that and how you think the reader will understand this. Your editor works to make your text clear.
Do you know how to shut up? First-draft writing tends to be slack, and revision may not fix it. An editor will know how to tighten your prose, identify rambling, drop verbiage, make your point more direct. Your editor works to make your text concise.
The main thing is that your editor will weigh what is appropriate. Is what you say appropriate to the subject? To the situation? To the occasion? To the publication? To the audience (the party frequently disregarded in these operations)? Your editor may have to be the person to tell you, tactfully, that you are not as funny as you think you are, as elegant, as impressive.
Your editor, if they are competent and professional, does not want to demonstrate superiority over you, but to assist you in accomplishing your purpose, to collaborate to make your text more efficient and effective, to keep you from making an ass of yourself in public.
Yes, you need to pay for this. Expertise as an editor is acquired by study and apprenticeship in the craft. Your friend who got a passing grade in English in high school is not an equivalent.
