Just as the various villages produce goods relative to their local climate and work force, the individual cities use commodities in accordance to their citizenry. Given that we have many sorcerers, thieves and seers, it's not that unfathomable that Thakria will fashion items for their use, eg knives, cauldrons, keys, all of which require tin, hence the city's desire to mine it, in order to help support those guilds.
Unlike Paladins, who simply need non-fighting rangers to pick poisons, or Craftmasters, who just need idling panners to get silver, our citizens require a more skilled and proactive servicing. Given the variance in local economies, I can certainly understand the confusion you have.
Written by my hand on the 20th of Paglost, in the year 1173.