Firstly, because the euro will become the pre-eminent symbol of economic and monetary union which is taking shape between sovereign states abolishing their national currency in exchange for a single currency.
Secondly, because when it is introduced, the euro will not immediately be both a unit of account and means of payment: on 1st January 1999 the euro will become the unit of account, whereas it will not begin to circulate as a concrete, physical means of payment until three years later, on 1st January 2002.