EPEL is like the "Fedora Extras" repository, but intended for use with RedHat Enterprise and CentOS.  Thus, by adding the EPEL repository to your production server, you have access to a LOT MORE pre-built (and tested) software packages that you don't need to worry about building all by yourself.  In my opinion, it's a huge time-saver, and reduces the risk of anyone on my team from injecting a compile-time configuration mistake.

All hail the Pre-Packaged Repository and the Automated Software Updates notifier -- without these, I'd be encumbered with managing updates several dozen tarballs of the essentials, instead of focusing on the interesting software problems that I'm being paid to work on! 

The EPEL Announcement Email