CSM is required for tools that aren't UEFI enabled. memtest86+ for example can't use a UEFI GOP and requires a BIOS capable VGA. Can't be used with a UEFI-only

CSM is required for tools that aren't UEFI enabled. memtest86+ for example can't use a UEFI GOP and requires a BIOS capable VGA. Can't be used with a UEFI-only video card (e.g. recent ryzen igpu). Also, there is still plenty of hardware out there that only comes with BIOS-capable option roms. These still require CSM if you want them to be be visible at boot.