Shooters allow you play out scenarios and kill "virtual" people without risking your own life. It's exhilarating being able to sneak up on someone, or snipe someone from across the map or get down and dirty! The best part is that even if you die, you'll still be able to do that stuff again; as opposed to real-life where you've only one life to lose. Ultimately, it's a very fun escape from reality and, to me at least, it's as close to war as I'd want to get. Also, as others have stated, it's a good stress reliever.