The need for currency conversion occurs when measure values have to be reported in a currency that is different from the one used to collect the data originally. It might seem as though this can be solved using a simple calculation; however, your assumptions might be wrong. Consider the following approach to currency conversion:
Collect currency exchange rates on a daily basis
Use daily exchange rates to convert each transaction into the desired currency
Why could this pattern be wrong? It might be correct in some cases, but it might be wrong for a variety of reasons. For example, the date used to link the exchange rate value to a transaction might be not the right one, because the date of payment is not recorded in the transaction itself. In another example, if the payment has been made to a foreign currency bank account, the conversion to a different currency should be made using a fixed exchange rate instead of a daily changing exchange rate.
Therefore...