You have probably played the game in Microsoft's version. However the simulator we are talking about is the one used by both pilots in commercial airliners use and astronauts to train in.
Pilots are trained in all sorts of weather and different mechanical and electrical failures that may arise in to be aware and know how to react to the situation.
All pilots have to be ready for the unforeseen. You never know what may happen or when so this is the biggest reason for a simulator. They are also used in schools to teach future pilots how to fly and get themselves familiarized with the controls and movement the aircraft would make in the air.
In some schools even Microsoft Flight Simulator is used to learn for the first time potential pilots. So you may want to look into buying yourself a copy of you don't already have one. The latest version Microsoft has is Flight Simulator X Deluxe Edition. You get the entire experience right from the comfort of your own home. Push backs, watching the ground crew work, talking with air traffic, and other traffic in the sky. Of course the game does require you to have at least 15gb of disk space, Windows XP or Vista with 512 mb, (256mb for XP but more is better) and at least a 1 ghz processor to run. I suggest using a top notch computer to avoid freeze ups while you are flying or even worse while you are trying to land. Nevertheless the program is perfect for learning to fly if you can't afford like most of us a real actual simulator like NASA uses.
