GRUB2 and other boot loaders which "understand" file systems must make assumptions about the character set encoding in file systems which do not store file names in UTF8/UNICODE, such as AFFS, SFS or HFS.
OK, I admit that I see little purpose to assign non-ASCII names to files relevant to the boot loader.
Nevertheless, separating /boot from / allows to use an ASCII/UNICODE-friendly encoding in /boot which Bootloaders will support in any case, and to use a filesystem with the weirdest possible LC_CTYPE encoding on / if one desires.
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