David Moher and colleagues underline how harmful predatory journals are in medicine and related fields (Nature 549, 23–25; 2017). In other research areas that are less well funded, such as my own field of palaeontology, my hunch is that these journals are irrelevant.
Like any other academic, I receive e-mails trying to entice me to publish in the New Journal of Dodgy Fossil Research or similar, but I suspect that — at best — these journals receive papers mainly from fringe researchers who are struggling to get their weak results published. I doubt whether anyone actually reads those articles. And if research isn't read, it doesn't exist.