It seems I get this question at least once a day. I’ve answer it a variety of ways, but it all comes down to one thing:

When your series is big enough to have fan art and screaming teenage girls falling in love with your characters, will they find a detail that changed somewhere along the line? Because you know they will. 

After all, you wrote half a dozen drafts before you ever sent it to a copy editor. The tattoo may have started out on your protagonist’s shoulder, but then you liked the idea of it being on her wrist. Now, half of the time it’s referenced, it’s on her wrist, and the other half it’s on her shoulder. I’ll help you fix that.

Maybe you continually lose your characters within the room while you’re focusing on the dialogue. He’s sitting down. Now he’s standing by the door. Now he’s leaning the chair back. I’ll help you fix that.

Or maybe the personal biographies of minor characters escapes your memories from one book to the next. After all, you’ve written a book for your other series since the last time you worked on this series. I’ll help you remember that the servant girl was reading in book two, so that you don’t teach her how to read in book four.

As a continuity editor, I keep a personal file on your series, continually adding details as you release books. Then, before the new book goes to the copy editor (what I do is best between Betas and Copy), I’ll make sure that everything lines up with the rest of the cannon.

I can help you avoid awkward questions from rabid fangirls later on. Or, you know, just get your manuscript to agree with itself.