Look at your own experience, the building phase in the design phase. We will (almost) never get a complete spec before hand because people are in charge and peo

Look at your own experience, the building phase in the design phase. We will (almost) never get a complete spec before hand because people are in charge and people change their minds. The sad reality is that building something with developers is often a faster, cheaper way to discover what is actually needed. Most people cannot tell you what they want until it is right in front of them and even then the response is usually, "Not this".