Why do you need a professional editor?
In short, so your work won't be trash.
Some people say they're able to edit their own stuff. Maybe that works and yay for them. But I've read books that were self-edited, or they should fire somebody! Sometimes I can't even finish the book because I'm stumbling over grammar mistakes and inconsistencies.
Apparently, I really like the word and! I'm also a huge fan of adverbs. I never have enough description or dialogue tags. I'm forever being asked, "What's it smell like? What's it feel like? What's it taste like?"
Readers will forgive a lot - they won't forgive consistent grammar issues. You've only got one shot at a first impression. Wouldn't you want to make the best impression you can?
Lucia Sugafield, learning how to write better.