This is especially true for teams of people. Somehow, as a project moves forward, small assumptions and well-intentioned but poor decisions accumulate, turning hours of work into a lousy user experience. The smart teams eliminate their mistakes before they ship by using a technique called UI prototyping. Combined with usability studies, prototypes keep teams headed in the right direction. Prototyping is a means of exploring ideas before you invest in them.
Aeronautic engineers use wind tunnels.
Bridge builders create stress models. Software and Web designers create mock-ups of how users will interact with their designs. The best reason to prototype is to save time and resources. Relative to the real product, prototypes are easy and inexpensive to create. So, for a minimal investment, you can find usability and design problems and adjust your UI before you invest heavily in the final design and technologies.