CentOS is a rebuild of RHEL. While it's rare, there are some bugs that creep in during that process. It also doesn't have a few other things that RHEL does - mainly the -supplemental repo (closed source 3rd party software, the most important of which is probably Oracle jdk in rpm form), a working yum-security module, and a slightly different update mechanism (both are yum, but if you are creating some mass deployment system then having subscription-manager is important; this is really edge case-y though).

And to be very, very clear - Red Hat does not support CentOS. Some of the CentOS devs are paid by Red Hat, but if you open a support ticket for it it will be closed very quickly. You cannot pay Red Hat for commercial support of CentOS in any way.

As a developer, it means I can now test my software directly on RHEL. CentOS is RHEL with different branding (and now supported by RedHat), but there might be small differences that creep in. Customers like hearing that you've tested it directly on their OS.