A simulation is a computer model that mimics the operation of a real or proposed system, such as the day-to-day operation of a bank, the running of an assembly line in a factory, or the staff assignment of a hospital or call center.
Time Based Simulation is time based, and takes into account all the resources and constraints involved, as well as the way these things interact with each other as time passes.
Matches Reality It doesn't always take exactly 5 minutes for a customer to be served and a customer doesn't always arrive every 15 minutes. Simulation builds in the randomness you would see in real life. So when you make changes to the simulation you see exactly how the system would behave in real life.
Effective Testing With simulation software you can quickly try out your ideas at a fraction of the cost of trying them in the real world. And, because you can try ideas quickly, you can have many more ideas, and gain many more insights, into how to run your organization more effectively.
There are many process improvements you can make using simulation software. For example, higher quality and efficiency from capital assets, better management of inventory, higher return on assets. But some of these improvements could be made without simulation.
So the real question is 'Why use simulation instead of another method?'