We've heard it many times..."if he has a knife I'll just shoot him". Oftentimes in the real world things always don't play out as we hope and if we're surprised by an attacker getting that pistol out in time might be a real challenge if not impossible. The knife can be employed very quickly and is easy to conceal and deploy. Many are very surprised how quickly one wielding a knife can close the distance in an attack. Here's a great video in which martial artist legend Dan Inosanto demonstrates the deadly effect of a knife.
Gabriel Suarez demonstrates in one of his force on force courses that oftentimes when gun and knife meet it's merely an exchange of gun shots and knife wounds...not much of a bargain! This video is very telling in how quickly a knife can be in your face:
In this last video we can see the frightening effectiveness of a knife. In the end the shop owner ends the conflict with a handgun but not before himself and two others are gravely injured. We can't just rely on our handgun to end a conflict. Situational awareness, empty hand skills are also essential to possess given the speed and surprise of a knife attack.