Costs, benefits, and criticisms

Large reserves of foreign currency allow a government to manipulate exchange rates - usually to stabilize the foreign exchange rates to provide a more favorable economic environment. In theory the manipulation of foreign currency exchange rates can provide the stability that a gold standard provides, but in practice this has not been the case. Also, the greater a country's foreign reserves, the better position it is in to defend itself from speculative attacks on the domestic currency.
There are costs in maintaining large currency reserves. Fluctuations in exchange markets result in gains and losses in the purchasing power of reserves. Even in the absence of a currency crisis, fluctuations can result in huge losses. For example, China holds huge U.S. dollar-denominated assets, but if the U.S. dollar weakens on the exchange markets, the decline results in a relative loss of wealth for China. In addition to fluctuations in exchange rates, the purchasing power of fiat money decreases constantly due to devaluation through inflation. Therefore, a central bank must continually increase the amount of its reserves to maintain the same power to manipulate exchange rates. Reserves of foreign currency provide a small return in interest. However, this may be less than the reduction in purchasing power of that currency over the same period of time due to inflation, effectively resulting in a negative return known as the "quasi-fiscal cost". In addition, large currency reserves could have been invested in higher yielding assets.