It forces applications to use the default language for output:
$ LC_ALL=es_ES man ¿Qué página de manual desea? $ LC_ALL=C man What manual page do you want? and forces sorting to be byte-wise:
$ LC_ALL=en_US sort <<< $'a\nb\nA\nB' a A b B $ LC_ALL=C sort <<< $'a\nb\nA\nB' A B a b 