1. Windows will leak. Not all of them, but over a whole building, it’s a matter of when, not if. We therefore need to detail our rough openings so that when a window leaks, the water can drain out harmlessly. (Note: this is why we place the air seal at the inside edge of the window, or door, assembly, to allow water to drain outboard of that seal.)
2. Origami is hard. The conventional way of flashing a window with papers and tapes depends on dozens of steps being performed perfectly every time: careful folds and precise manipulation of less-than-forgiving materials in the field. Simple mistakes, like reverse lapping, can be catastrophic to the assembly but can be covered up by subsequent layers of material, so checking work thoroughly can be impossible. Now multiply these risks by the number of windows on a building and you will understand why window installations can be anxiety inducing.
3. Flashings should be vapor permeable. If flashings are not vapor permeable then moisture can build up behind them and cause rot. The high vapor permeability of fluid applied flashing ensures that construction moisture and seasonal water vapor migrating through the wall assembly does not accumulate behind the flashing and can readily dry.