"Good editors are your partners in making good comics. They are on your side. They are the trained second pair of eyes who look at your stories without the baggage you bring to them (sometimes, sadly, artists can get too close to the comic they’ve been working on for years, and not see the occasional story snarl or character breakdown), and challenge you to make your comic the best it can be. They point out when a comic panel doesn’t read properly, when a character looks off-model, and sometimes they’ll even give you notes like 'this looks awesome!!!' with little hearts drawn in the margins. Good editors are worth their weight in gold."
— Faith Erin Hicks, on why she prefers to work with an editor
Of course, I'm biased because I was an editor before I was a writer, but I think Faith hits the nail on the head here. Everything is improved by a second set of eyes. The whole piece is worth reading, because Faith talks about the sort of discussions she has with her editor and what she is and isn't willing to change.