A Dyson sphere around a black hole

The whole concept of a Dyson sphere is kinda idiotic. Any civilization that is capable to build one, it is probably able to work out fusion energy very efficiently.

There is no point to go and harness energy around a star or a black hole, when you can just produce it locally with a lot less resources/waste and materials. The sun itself is actually very inefficient in producing energy.

There is no need to harness the sun million of km away, when you can recreate it in your home planet. The only way to produce a dyson like of sphere, is to tame an over-heated sun, and reflect away un-needed energy. But there is no point to build one to just harness it.

It makes great sci-fi stories, but that's about it. Scientifically, it just doesn't make sense.

It depends on the amount of energy the civilisation needs.

Lets say, you want to manufacture new planets/planetary sized spaceships. You would need very big fusion reactors and you probably want to save the special fusion fuel for those, when they are on their voyage to other worlds. Because then they need fusion.

But why build a fusion reactor, when you already have a big one avaiable?

Because you can build a more efficient one? You could make it portable, controlled, modular etc etc