The idea is that if a machine defaults to "legacy boot," meaning a BIOS-style boot, then use that to load U-Boot, which then provides a software emulation of UEFI so that the startup process can be simplified by the removal of BIOS support.
The last time I actually tried anything with Redhat I was trying to build a file server with RHEL v6.8 on a circa 2014 Dell. Absolutely zero support for the drive controllers. It felt like installing Linux in the mid-1990's. I gave up in frustration after two days and gave Ubuntu 16.04 LTS a try. As far as I know, that server's still chugging away with 98 terabytes of storage at that office.