Even when it's practical to place students in real-life situations so that they can learn by doing, it's not always preferable.
Simulations offer two key advantages over real life.
The first is that real life tends to keep marching on by. Simulations allow students to play with time in ways the real world does not permit. Often, the real world moves so quickly that students do not have time to think things over as much as they would like. However, in a simulation, if a student wishes to sit and ponder his course of action, he can freeze the simulation, and perhaps even ask an expert some questions. If a student is unclear as to why things turned out the way they did, we can allow him to loop back in time and review the course of events.