The foreign exchange market is called forex. If you exchange greenbacks for euros at you bank, your bank bundles your exchange with other transactions and trades them on the foreign exchange market. The idea is to get the most favorable rate of exchange. In this way your bank intends to make a profit on your transaction. Forex exists to help international investments and trade. If you went to Europe with dollars, you couldn’t spend them. World firms have an identical issue, so currency exchange exchanges the currency.
