This expression, I had thought, refers to writers who assist, to greater or lesser degree, celebrities and politicians to write their memoirs. Their lives and the stories they have to tell are interesting enough to sell a lot of books but the individuals' talents do not include crafting a memoir or autobiography. A person with this skill is brought on board to assist. This seems reasonable to me although I hope the person in the background gets some credit for their abilities.
What I was surprised to learn recently is that best selling writers, who worked their way up to their status by acquisition of skills in addition to their native talents, are using ghost writers to augment their own production. James Patterson, who was, I believe, the highest earning author last year cannot personally keep up with the volume of new works that could be sold and generate profits for himself and his publisher. There are many who would purchase a new book monthly, if one was available. The solution to this has been to bring ghost writers into the mixture. This must be a unique skill set for not only must the person be a competent writer in their own right, but they must be able to suppress their own natural style and voice and assume that of Mr. Patterson. Is that possible? He describes it here: James-Pattersons-Kentucky-fried-books
Actors, particularly talented actors like Meryl Streep, can bring characters to life in every detail. I've read that she studies videos of the person she will be playing, for example, Margaret Thatcher, by the hour and uses voice coaches to capture the nuances of speech and accent. But, it seems to be that a writer who can do this should be a best-selling author in their own right and under their own name, not lurking in the shadows.
I recently read that another well-known author, Wilbur Smith, admitted frankly that at age 79 he couldn't keep up with the pace of production his publisher and public would prefer. I can sympathize with that. He also is now using ghost writers. I suppose he and the others writers who employ this production technique review and approve all work done in their name. That must be disconcerting; to see yourself, or at least your writing, as others see you.