Region planning is the unglamorous little engine behind convincing maps. Before a polished map exists, there are questions about terrain, settlement patterns, trade, distance, danger, and why a road or border exists where it does.
A dedicated sheet gives those notes somewhere to live beside sketches. That makes it easier to see contradictions early, while the map is still cheap to change.
The aged-paper treatment keeps the planning documents visually connected to the finished world. It turns development notes into something you can keep, print, or hand across a gaming table without losing the atmosphere.