Imagine yourself in a dark room, struggling to make out the outline of an offender, when there ia a sudden flash of brilliant light in your face. Almost like a camera flash. That will be the last thing you see for several seconds. Meanwhile the blinded offender can still see the spots from where the light eminated, and will likely attack in that direction, if still capable. An AR-15, especially the carbines, emit a brilliant flame from its muzzle when fired in dim lighting. The flash hider mainly protect you from being blinded by this flash.
Among other things, if you ever shoot with magnified optics, you can notice even on overcast days, a flash that momentarily obscures sight of your target. Using the hider allows you to better track your target. The flash hider has a multitude of benefits, and the only major drawback is that some believe they caue a slight degradation in accuracy. The vortex, however, has been noted to improve accuracy on some barrels.