Openrange Defense: In an emergency, it isn't over until you verify there are no other threats. Scanning accomplishes this by doing the following:
One of the skills that we introduce in our Self Defense Pistol Skills Level 1 class is 'scanning', meaning that after you shoot, prior to holstering, you 'scan', or look around. Specifically, you are looking for additional threats. According to UCR (Uniform Crime Reporting), if you find yourself being subjected to an armed robbery, there is a 40% chance that there is another person involved...even if you don't see them.
True story. I spent 10 years of my former law enforcement career in Memphis, which is a high crime city. Armed robbery was of course common, but private citizens legitimately defending themselves in such situations was also common. One individual found himself in a parking lot, facing an armed robber, and the citizen shot the robber in self defense. Unfortunately, the robber's accomplice came up behind the citizen with a shotgun, fatally wounding him.
The reality of armed self defense nowadays is that you may have to defend yourself in a Kroger, a Walmart...or a parking lot of one of those stores. Here's how it works on the street: If you are approached in such an environment by an individual bent on armed robbery, that person is likely the passenger of a car nearby. After successfully robbing you, that robber is likely to run, get in the passenger side of the getaway car, and takeoff.
Some shooting schools simply have you do a slight head shake, or 30 to 45 degree scan to the front. That's not enough. If you were just involved in a self defense incident, you are in Holy Crap mode, with emotion and adrenaline forcing you into Tunnel Vision: At this point, you want to take your finger off of the trigger, keep the pistol pointed in the direction of the last known threat, and LOOK around you...180 degrees to the left, and 180 degrees to the right. Adding this to your training helps ingrain the habit, and having this as a habit in the real world just may save your life.