Many authors wonder why or if they even need an editor? I thought this before I hired one for my own manuscript and before becoming an editor myself. The answer is the forest for the trees. You spend so much time cultivating your individual little trees you forget about the forest that makes up the entire work, not purposely of course. After that third revision pass, you have read your manuscript so many times you miss things that would be glaringly obvious to someone else. The reason for this is you have the entire story in your own head as it "should be".