I won't comment specific placement (or "development") agencies, some are serious, some are scams. Such agencies are usually not directly marketing models, i.e. they don't have bookers and clients like "normal" agencies. Their main role is to scout, (more or less) develop and place new faces with "normal" agencies, typically also abroad. The word "Development" is often a tad exaggerated as if there are no clients there aren't that many development opportunities and a few tests isn't what I'd call development.
Scouting networks are a major new face ressource for major agencies, it's more efficient than open calls and randomly submitted material (be it uploaded online, e-mailed or by regular mail).
Blunier