- For the companies who offer the certs - people pay them to test to prove they have skills, then tie themselves to using that company's product. Free organic marketing.
- For consulting firms who are business partners with the company who offers the cert. More certs = better partner programs.
- For people trying to get their first tech jobs: A cert with no experience is better than no cert and no experience.
But if you are a dev, just doing your work... they are at best a training tool, at worst an outright scam.
Then read once cover-to-cover the PMBOK. You will be tired. Your eyes will hurt. You will rethink some/many choices of your life!
Then read a 'normal' study-book on PMP, I checked out a few but I ultimately picked from the "Head First" series. This was a fun read, made sense.
Then read once more cover-to-cover the PMBOK.
Then go and rock that test!