Why do some countries pursue trade surpluses?

  The same reason why banks continue to hoard gold even when they refuse to sell the gold at a pegged exchange rate. I suppose it is simply out of tradition. Everything is fiat money now, thus most monetary decisions are inconsequential. Mercantilism was a common theme prior to the introduction of fiat money on a global scale. A trade surplus often meant you were allowed the means to withdraw gold and silver from the nation posting the deficit at a pegged rate. Unlike today, the market price for commodities would actually rise if there was a sudden explosion in buying. Under a commodity standard such as the gold standard, the price of gold would remain stagnant regardless of trading behavior. This helps lend to the credibility of a nation's fiat currency because it remains solid, even against commodity prices.

  Thus nations that adhered to a gold standard were risking having all of their gold withdrawn not at market price, but at a pegged exchange rate if their imports exceeded their gold reserves.  Today, no such relationship exists between a nation's currency and its gold reserves. Just like a bank will never sell gold to its depositors, a nation-state does not sell its gold reserves in exchange for its own fiat money. As a result, there really is no incentive to have any sort of trade balance.

 In fact, manipulating the currency to become overvalued so as to import the world's commodities and then letting the currency value collapse would actually be a profitable venture. The exporter of those commodities would then be holding depreciating assets in the form of fiat currency, while the importer of those commodities are now in possession of an appreciating asset.