Whether you consider eliciting requirements to be a separate activity, or a part of systems analysis, the importance of correct requirements must be a high priority for you. Building accurate models means that you can guarantee the correctness of your requirements.
All engineering disciplines use models to develop the products they intend to build.
Requirements models are used to discover and clarify the functional and data requirements for software and business systems. Additionally, the requirements models are used as specifications for the designers and builders of the system. What a system is and what does a system do You can describe a system by what it is, and by what it does. Beyond this innocent description of what the system must do, lies a complex set of rules, procedures, data and functions. It is the task of requirements modeling to discover the rules for calculating the cheapest fare, the algorithms needed, and the data needed to support those calculations.