Uses of string-related functions would doubtless be myriad. One in particular
that would be handy, from my point of view, is checking whether input to a
hatnote contains "[[" and "]]": if it does, then the wrapping "[[" and "]]"
could be dropped. This is useful because not requiring [] in the parameter
input has become the norm on enwiki, and this makes it impossible to replace an
intended single link with "[[Link1]] or [[Link2]]" due to parsing oddness. If
StringFunctions were installed, "[[Link1]] or [[Link2]]" could be used as the
parameter, and the template would drop the default brackets.
Okay, so perhaps that's a slightly pathetic reason, but there's no request open
for this yet, so let others add their own reasons.