A good editor is worth their weight in gold. Myself, I’m a frugal person, so I do my own ferocious self editing. I once belonged to a writers group, and the material the members submitted for critique made me cringe. Their story ideas were good, but were full of unnecessary adjectives, numerous misspelled words, run on sentences, and they had their characters grunting, snorting, huffing and puffing. I would spend time editing and when I gave back the material, the people were offended. I figured these people were looking for praise and not the truth.
ROUND TWO: an in-depth, comprehensive, line-edit that includes rewriting sentences, improving word choice, organization, plotting and pacing. We’ll make the dialogue stronger and more believable, the action more tense and nail-biting, and the emotional peaks less melodramatic and cheesy. For non-fiction, we’ll strengthen your argument and make the writing more engaging.
Obviously there is a big difference. I actually started with screenplays (with some mild success) partly due to hearing that as long as you have a good story, the grammatical errors won’t be a big deal. Well, that was not true. Maybe if a producer liked the story or I was already established but every contest I entered always came back with feedback about the grammatical errors. Even if I won or placed as a finalist, I always received critique on that. Very little is spelling error, it was more about structure, etc.