Designing without a purpose is failing slowly.
Prototyping is a pretty hot topic within the design world. Most of the time, things veer off into discussions about tools, techniques, or methodologies. You only need to know two things about prototyping – when to start and when to stop. Start when you have questions like “what if” and “how about”. Stop when you have to choose between two desirable, but incompatible options.
Decision-making is inextricably linked to design, and prototyping is most effective when you and others can see what the choices are. Otherwise, you’ll just find yourself talking about your options instead of evaluating them.