Ultimately, a developmental editor helps you develop as a writer. Sure, they help you write a story that others will want to read, but supporting your improvement as a writer is at the heart of the job. What you learn from a DE for one story should help you as you write future stories.
One of the most important lessons fanfiction writers learn as they write and post their stories, is that having a beta they can rely on for constructive criticism can mean the difference between an ok story and a story that winds up on recc lists long after their favorite show is off the air.
As a professional editor, I want to help other fic writers disprove the notion that fic writers aren’t real writers. We know better. Plenty of fic writers have published original fiction, some even winning awards for it. You can, too, and I can help you.
Last year, I plowed through the bulk of Kerry Greenwood’s Phryne Fisher mysteries. I devoured the twenty or so books in the series like I’d hit the Girl Scout cookie jackpot (which I have no doubt Phryne would approve of on several levels), only with none of the guilt or potential diabetes and a few bucks left in my pocket.