I have a vague notion from reading science fiction stories that black holes may be extremely useful for highly advanced (as in, post-singularity/space-faring) civilizations. For example, IIRC, in John C. Wright's Golden Age series, a colony formed near a black hole became fantastically wealthy.

I did some googling, but all I found was that they would be great at cooling computer systems in space. That seems useful, but I was expecting something more dramatic. Am I missing something?