They emit energy
The Sun is an excellent source of energy. However, this is nothing compared to the energy emitting from a black hole. Statistically, black holes convert 10% of their mass into energy which is a lot compared to a simple nuclear fusion which only converts about 0.7% of its mass into energy.
There is a stronger gravitational force at the inner event horizon, causing matter near this point to orbit much faster. Once the matter accelerates, it heats up, going over millions of degrees Celsius which is enough to turn mass into energy with the help of radiation. Many researchers are looking to use ‘blackbody radiation’ to give power plants and starships energy.