Computer simulation program is used so as to explore as well as gain new insights into new technology. It is of purpose as it approximates the performance of systems that are too difficult for analytical solutions. It can also be referred as a computer model or computational model. Check out http://en.wikipedia.org/wiki/Computer_simulation for details.
Computer simulation is used in cases where if we did the experiment for real someone will get hurt or it is just too expensive. For example: They test new pilots in a flight simulator before they let them actually fly a plane.