Ryan Singer shares some really good insights into product design derived from his experience at 37 Signals over the last 10 years:
Deciding which situation you are designing for focuses the problem. Focusing the problem focuses the team. When the team knows where the value is, they stop speculating about all possible cases and they get down to business.
A product concept gives you a map of the territory you are trying to cover during implementation. Looking at that map shows you which areas you covered and which ones you haven’t yet. It helps you answer “are we there yet?” with more precision.
The goal of the design process is to reach fitness between problem and solution. The job defines the problem and your concept defines the solution. The moment of truth happens when you fit the two together.