In a recent video I saw while cruising the internet superhighway, I came across an annoying guy with a limited vocabulary who decided to spend a few minutes harassing a border patrol agent in his fully marked vehicle.
The issue was that the officer had his firearm drawn as the unknown man walked up to the car. From a practical use of firearms use this is a great discussion starter on good tactics and reasons for being aware of your surroundings, ESPECIALLY in a car where you're at a position of disadvantage. This is applicable COP OR NOT.
Judge for yourself as to whether or not this guy even deserved this much of an answer. Hopefully I shed a little light on why a cop would pull his gun as you walk up to him in a car.