But how does that help the customer (be it internal or external)? How is he supposed to align capital, human or financial? For example: when does he tell the marketing team to start the big launch campaign, etc?

An example would be something like a main-stream game, say Battlefield 3. The have marketing videos months in advance, they sell games in advance and by doing so they promise that the product will be available at a certain point in time. It may contain bugs, but, unlike what you say, it cannot just be whatever is ready at that time (say without support for sound, or not controls)...