If I understand you correctly, string would need to be burninated too – it cannot stand alone on a question (on Stack Overflow), and it's a concept present in a whole bunch of languages. Same for arrays in a way. But these tags do help, in my view, to get a general idea of what questions are about.
The tag complements questions about
readdir pretty well, whether they are C, Perl or some other language altogether. And it is a tricky function (in C anyway).
It makes searching for issues with that function more efficient (compared to searching for just "readdir", which will hit all questions that use that function in related code but aren't about
I don't see how this tag (or opendir/closedir for that matter) hurts.