And in Westeros, the Lannisters have the cube of gold and the Tyrells with the rich farmland of the Reach have the real resources. You can’t eat gold. You can’t feed it to your horse either. Gold doesn’t keep you warm during those lengthy winters. Gold is useful primarily because it’s a convenient medium of exchange (who wants to carry all that wheat around) and a durable store of value (keeping a whole bunch of horses alive and healthy is itself a resource intensive process). So people with claims over valuable real resources will often end up accumulating gold. But though the Lannisters have more gold than anyone else, that’s not how they got their gold. They just own gold mines.
Don't Believe the Hype—House Tyrell Is Richer Than House Lannister [Slate]