Rather, these black holes are more like an interesting anatomical piece. Stars and gas in galaxies have too much angular momentum to fall in - they'll just keep orbiting. Some gas and stars that get too close do get disrupted and eaten from time to time though.
And occasionally, they do use their intense gravity to generate powerful emissions from active galactic nuclei. Some supermassive black holes in some galaxies are actively feeding on gas close to them which generate powerful emissions, which we call quasars. Friction in those disks of gas heats the gas, generating the emission we observe, and causing the material to lose energy and angular momentum and fall into the black hole. This results in a pretty small change in the black hole's mass, so it's not like the event horizon is rapidly expanding outward and feeding at an exponential rate or anything apocalyptic like that.Source