As a writer, I know how hard it is to self-edit and proofread my own writing. It’s important to have my manuscript as close to perfect as I can make it before I send it to my editor. (Yes, writers who are also editors hire other editors to edit their work.)
As an editor, I know how easy it is to get caught up in a client’s narrative and miss both small and large problems that must be addressed. That’s one reason I always sub-contract proofreading to someone else. But I also make sure I’ve done my own due diligence before I pass a manuscript–mine or someone else’s–to a proofreader.