I think that this relates mostly to OEM rigs, and upgrade video cards, or at least, the cards from 2-3 years ago, that may not have had UEFI-compatible "GOP" BIOSes onboard.
See, the thing is, OEMs ship their UEFI Windows PCs (Windows 8 and onwards), with "Secure Boot" enabled. Sure, most of them let you disable it, but it's much more common on OEM branded rigs, than custom-built rigs.
And, in order for an add-on dGPU to function with a UEFI BIOS set to Secure Boot mode enabled, it HAS TO have a (UEFI) "GOP" (Graphics Output Protocol) -compatible BIOS installed. Or else, it won't work, or the machine just won't boot. At least, until you disable Secure Boot.
Edit: Thing is, most custom-built PCs / motherboards, that have UEFI, default to "CSM enabled" (compatablity support module, AKA legacy BIOS interface stuff). So ancient video cards, that don't support UEFI GOP, will work, as long as CSM is enabled. (Just not with Secure Boot enabled.) And newer cards, are GOP-enabled, so they'll work with UEFI BIOSes anyways.
And, hopefully, newer video cards have a bi-compatible BIOS, that works with both older non-UEFI mobo BIOSes, as well as UEFI GOP.