It is like the schematics, not the specs, of a thing, and a way to prevent time bombs later when real implementation work occurs. It is the stuff behind the walls. The possible locations of the walls. Not the beautiful stuff that covers up the walls and gets in the way of quickly understanding what needs to happen.
And when the effort to create something is small or low-cost (i.e. lightweight), people are more willing to discard it if it’s not quite right—because doing so doesn’t feel like a loss. In fact, throwing it away can feel productive, because it clears the path to a better solution without much sunk cost.