Light-weight prototyping is an important way to ensure a project's stakeholders are envisioning the same goal. It allows you to quickly test that the target user's needs are met. A simple PowerPoint prototype, a series of screen mockups, or an HTML demo can quickly expose mismatched expectations.
The same demo can inspire a brainstorming team to quickly clarify big-picture goals as well as important interaction details.
As an example, one of my prototypes clarified that a design with a "clear initiative" was actually universally loathed by the stakeholders. Better to find out early in the process with the least investment!
