There are three things here:
- Older BIOS firmware in a PC needed a MBR in order to boot. It looks at a specific place for boot code and excecures that. MBR+FAT and MBR+NTFS should work both since the firmware loads only the MBR/bootloader paret. After that is is the bootloader which needs to understand the filesystem.
- Modern PC firmware used UEFI. EIFI does not use a bootloader. Instead if looks for an EFI System partition (ESP). This partition must be in a format which the firmware understands. It is mandatory to understand a format which just happens to be 100% compatible with FAT.*.
The firmwate that either starts a boot program, or if none are set fals back to /boot/bootx64.EFI.
Now as I understand you question it is:
- I have a PC with UEFI firmware (and not BIOS firmware)
- I want to boot in UEFI mode.
Why do I need an EFI copmpatible fielsystem (FAT).
The answer should be clear with above background :)
And for completeness sake:
- It is perfectly possible to load a Linux installer on an ext2 formatted pendrive with MBR in an older BIOS system or in an EFI system with ancient backward compatability shims enabled.
- BIOS(UEFI mode) does not exist. Firmware (BIOS), firmware (UEFI) exist. And this can be firmware (UEFI) with CSM enabled, sort of acting like BIOS. It is not BIOS however. And often the shims are incomplete.
*: The firmware is allowed to understand more format. And the EFI on some apple systems understands HFS. But most systems implement the bare minumum. Which happens to be a rebranded FAT format.