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
