Make your product idea real and testable.
"A picture may be worth a thousand words; but a prototype is worth a thousand meetings."
Photoshop files are no longer the most efficient way to express web design. Creating a prototype helps teams have a live, moving design of their idea faster, rather than waiting for final art and web development to finish before you can check whether the design works.
A prototyping workflow helps your team document and build the design -- e.g. Which pages lead to others; What creative solutions to try to increase leads.
"...know our customers' needs, ensuring that our product looked great from the very beginning"