Re: "Certified"
One problem with RHEL it's that it's license activation process adds complexity if you want use it in automated systems like CI/CD. In those cases having the possibility to throw a CentOS at them and be done with it, with the confidence that your test environment will work the same way as your production system it's a godsend. Also, having local mirror of the repository is a pain, but with CentOS it was easy.