Some black holes exist in binary systems with stars, pulling matter from their companion. This infalling matter forms an accretion disk and emits X-rays and gamma rays, making the black hole detectable.
It matters because X-ray binaries are key to discovering and studying stellar-mass black holes.
It also allows scientists to estimate black hole mass and spin.
Some black holes actively feed on a nearby star in binary systems.
[NASA, nasa.gov]