Too Long; Didn't Read
For practically anyone, unplanned work kills several hours of planned productivity. For creative workers, such as those who write software, it kills days. When the only definition of “done” is “the customer said they were satisfied with the analysis”, you know the scope of your project is going to forever creep until the customer decides to pay attention to something else. When working on something creative like writing code, you experience different levels of productivity. The most productive levels are what some people refer to as “being in the zone”