the accretion process is one of the most efficient ways of converting mass into energy (that we know of anyways). suppose a very small amount of matter undergoes a chemical reaction of some sort (combustion, synthesis, acid-base, etc.), and releases several hundred eV (electron volts) of energy. if an identical amount of that same matter were split apart at the atomic level (nuclear fission), it would produce several hundred M
eV (several hundred million
electron volts). the rate at which nuclear reactions convert mass into energy is several orders of magnitude greater than any chemical reaction. in nuclear fission, approx. 0.04% of the mass involved in the reaction will be converted to energy. nuclear fusion is approx. 8 times as efficient as fission, converting approx. 0.3% of the mass involved in the reaction into energy. but when it comes to converting mass directly into energy, hardly any other method appears to be more efficient than accretion around a highly compact massive object (the only other method i can think of that is theoretically more efficient is matter-antimatter annihilation). while i don't have a source in front of me, i seem to recall reading that an accretion disk can convert up to a whopping ~10% of its mass into energy.
now, as AbsoluteZer0 mentioned, the radiation emission is triggered by friction within the accretion disk, which is itself caused by the central body's gravitational pull. now while a supermassive BH is much larger than a stellar mass BH (both with respect to mass and EH diameter), it is still confined to what is otherwise an extremely small volume of space in comparison to the size of the galaxy in which it is at the center. using the concept of energy density, one can easily see that the release of such prodigious amounts of energy from within such a confined volume results in an extremely bright source...in this case, it can sometimes be bright enough to be seen from across the visible universe.