Because investors with large amount of money to invest would be wise to diversify their holdings to reduce their currency risk exposure. This is definitely needed by Central Banks to hold their balance of trade surplus. For example, China, which earns shiploads of greenbacks due to their exports to US would be stupid to keep all the $ that we (Fed) can depreciate and render them worth less (as a way of settling our National Debt fox example!). By diversifying their reserves into Yen, Euro etc, they will reduce the risk.
