It, like the firearm, is a last-ditch action to stop, or eliminate a deadly threat to yourself. Using empty hands against a person with a knife is just plain stupid. If you have no weapon, and you see a knife . . . RUN! THAT is your defense. A firearm is the only real defense against a knife . . . if you see it in time, and/or can gain distance to draw and fire.
This video contains graphic video footage of real knife attacks . . . not staged activities.