It's too increase accuracy and look cool at the same time. in some games sniper rifles are inaccurate as hell when they aren't scoped, so when you scope in and shoot and pull out in less than a second, you have a chance to kill someone with speed and accuracy. Of course it takes a little practice. :V

^exactly. i nao own cod4 ( :3 ) but quickscoping is alot harder than in cod2.... maybe posting a vid soon.