Business requirements are usually expressed in words. System analyst find it useful to translate those words into picture called system models to validate the requirements for completeness and consistency.
or in other words,
A picture of a system that represents reality or a desired reality. System model facilitate improved communication between system users, system analysts; system designers and system builders.
During system design, analyst convert the description of the recommended alternative solution into logical and then physical system specification.
One must design all aspects of the system from input and output screens to reports, databases and computer processes.