I suppose this same idea applies to non-web apps as well. I once designed a product "software project survival guide-style" and wrote the user guide before designing the UI or infrastructure so I could focus on how users would accomplish their goals. It was an interesting (and fairly successful) experiment.

Quoted: In web design, when we think about flow we usually think about “task flows” or “flow charts” but there’s another type of flow that we should keep in mind. It’s that feeling of complete absorption when you’re engaged in something you love to do without being disrupted by anxiety or boredom caused by tasks that are confusing, repetitive or overly taxing.