The faster you rub, the warmer you get. Well, the same thing happens in the disk around the black hole. The stuff closer in moves faster. However, it's also rubbing against the stuff just farther out, the stuff moving a bit slower. Because it's rubbing so hard, the material in the disk gets very hot , easily into the millions of degrees.
Hot objects glow, and very hot objects tend to glow very brightly.
As a matter of fact, we think that quasars , intensely luminous objects that are extremely far away, glow from this very mechanism.