Discrete-event means that we view a system as progressing through time from one important event to another, rather than changing continuously. Many systems can be viewed as queuing networks, which is well supported using this approach. For instance, a customer joins a queue at a discrete instant of time and at a later discrete instant leaves it. This view makes for a computationally efficient way of representing time. The ability to represent time is the second great strength of simulation.
Simulation means making a simplified representation of an original.
Just as a model aircraft captures many of the important physical features of a real aircraft, so a simulation model captures important operational features of a real system. Simulation software is built from entities and processes that mimic the objects and activities in the real system. Decision support is the short answer. Obviously, you want to make the best decisions possible. It is often impossible to study a system in any other way because the real system hasn t been built yet.