A prototype is worth a thousand words
Too often assumptions about design, features, and performance get written into specs or delivered to users without being tested first, resulting in bloated requirements documents that cost too much to deliver and applications that don't meet users' needs.
Prototypes let you quickly and economically test your assumptions about how things will work before they get baked into specs, or worse, your application gets released and no one wants to use it.
first prototypes can usually be delivered in 3-4 weeks
range from clickable designs to a thinly-implemented vertical slice of your application
sell your vision to the business better than a powerpoint