Benefits of Use-Case Modeling

Sometimes people have a hard time understanding what we as programmers do. Why certain things cant be done and why other things that seem complex are actually simple. When you are working on a project to develop software or systems, you may use UML. How would you describe UML to someone who doesnt have any experience working on the project team? How would you describe the difference between logical and physical system models?

There are no right or wrong answers here, but be sure to support your position.