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.
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.