Think of it this way: Regular book agents are "employed" by authors and would-be authors. It's their job to place a book that they think is promising (often only on the basis of a book proposal) with a publishing house. But literary scouts work for such outfits as movie companies and foreign publishers. It's their job to ferret out the books that are being hotly contested by publishers, to anticipate what might make next year's big movie or a hot-selling book elsewhere. To that end, they stay in touch with editors at U.S. publishing houses, always looking for the latest proposal that's already the obscure object of desire...or maybe not so obscure. But the best target is a book that hasn't yet been bid on by publishers.
So why would editors share information with scouts about hot new projects? Williams quotes Houghton Mifflin Harcourt Publisher Bruce Nichols:“It’s an information source for editors,” he explains. "It’s information you might not get elsewhere from someone whose self-interest is not so direct in any given project.”
Trading in information is an old profession--sort of like tinker, tailor, soldier...literary scout.