I did some investigation into this enterprise a while back, in response to an invitation to review a paper for Frontiers in Antimicrobials, Resistance and Chemotherapy. That post concluded that the enterprise is basically a pyramid scheme, drawing in scientists to create ever more journals and staff their editorial boards. A few journals are quite successful (the spammer Frontiers in Neuroscience is one), but most attract very few papers - usually there's an opinion piece by the Editor, and maybe a paper from an Associate Editor's group, but that's about it. The editors are (of course) unpaid, but the publication charges applied to any papers that do get published make money for the enterprise.
Ironically, the email I just received starts with this:
Dear Dr. Redfield:Perhaps he meant my post about NASA's arsenic debacle.
I came across your blog post and was very impressed with it! I am now writing to request your assistance: I have accepted an invitation to serve as Specialty Chief Editor of a new journal entitled Frontiers in...