The UML for Systems Engineering
People construct models because they are far simpler and cheaper to build and test quickly than the actual thing being modeled. Most models are fairly special-purpose, and omit details not relevant to this singular mission. Scale mock ups are not good for showing electrical conduits and an air flow model won't help with the analysis of the structural integrity of the building. However, by building such models we increase our chances that the product to be constructed will actually meet the needs for which it was commissioned.