The essence of simulation is the building of a computer model rooted in a real-world applications, one that allows the freedom of experimentation within the safety of simulation. In most cases, the building of a model on a real life scale is near to impossible or extremely expensive.
Simple example Let's imagine the bank with several cash desks.
The initial parameters of that system are arrival and service rates, the number of cash desks and simulation time. Simulation model is not mathematical. It simulates events of real systems. In comparison with mathematical model the simulative one allows to overcome the mathematical limitations. It allows building models with complicated logic.