[cletus]

There are actually a lot of theoretical applications.

Black holes are likely the most efficient form of power generation that we currently know of.

Black hole propulsion may well be the best form of interstellar starship propulsion.

Black holes may also be the ultimate computer.

If the universe continues based on it current understanding then eventually all the stars will be dead and the universe will be a dark place. The Black hole era will last trillions of time as long but may well be the golden age of civilization.

Each of these topics is a rabbit hole.