Why Prototype?

I ran across an interesting presentation (”Sketching in Code: Using Prototypes to Visualize Interactions“) that David Vreba, Director of Technology at Adaptive Path, delivered at the UXWeek conference in August.
In particular, two slides in David’s presentation with the titles “Why Prototype?” caught my attention.  These slides sum up in a nutshell why prototyping is so important:

Visualize your requirements [...]