Roof, Shop, Store
Some of your private dirt is on the ground, so you stick a flag into it. Then on this private land you claimed, you build a roof. Under the roof you also build some shops and stores.
That's the natural next progression of claiming some land. Building a roof upon it and building some shops and stores inside. You'll see why in a moment.
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や
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The roof covering the dirt is there so that the Yankees (や) can play baseball year round. The stores and shops are there to sell hot dogs and Yankees memorabilia.
You have yourself a whole Yankees stadium, and all it took was a roof and some shops / stores! If you build it, they will come.