SISO exists primarily because major users of modeling and simulation (M&S), including the US Department of Defense, have a massive and expensive problem. They use M&S for many purposes, including:
The field of operations research arose to address military applications, and there is still a major emphasis on using M&S to study the extremely complex problems of planning, organization, logistics, doctrine development, assessment of proposed new systems, etc.
Research and development
At every stage in the development of a new vehicle, weapon system, command and control system, etc., M&S is used for tradeoff studies, design decisions, performance prediction, logistic support requirements, among many other uses.
Test and evaluation
As new systems are developed, they are tested first in M&S before they are taken onto physical test ranges, which are very expensive to operate. Interfaces with other systems are studied using M&S, and often other systems with which the new system must interact are represented in testing by simulations of those systems.
Doctrines for how to use new systems are usually developed and tested via M&S. Then training simulations are developed for those who need to learn how to operate, support, maintain, or otherwise interact with these systems.
The "expensive" aspect of the problem arises not only because of the complexity of the simulations themselves, but also because of the many different types of simulations that need to be developed during the life cycle of a system. By and large, models and simulations of a new system are developed "from scratch" by each agency and contractor at each stage of system development, implementation, and use. Historically, there has been little re-use of simulations across systems or across phases of development, so taxpayers have often paid for the creation of multiple simulations during each major system development program.
A related issue is the complexity of these simulations. In addition to making the simulations expensive, their complexity ensures that very few people have any deep understanding of the behavior of the simulations or the assumptions that go into developing them. In some cases, this lack of understanding, and the tremendous time and cost involved in developing a new simulation, have resulted in simulations being used for purposes beyond their scope of applicability. Sometimes this occurs as the result of a conscious decision to use an existing simulation rather than developing a new one; in other cases, it occurs as a result of lack of understanding of the limitations of a simulation.
Since the late 1980's, there have been serious efforts to address the related problems of expense and complexity by encouraging the development of simulations that are more modular and reconfigurable so that they can be reused for multiple purposes and multiple programs. SISO (and its predecessor organization) have played a major role in these efforts.