You realize gains when you use another currency to buy something. So if you exchange USD for Euro and then the price of Euro goes up by 5% then when you buy a sandwhich with that money you have to pay taxes on 5% of the price of that sandwhich. Unless you convert your USD to Euro at the time of the transaction buying things with euro can turn into a tax nightmare since you have to keep track of the exact time you purchase something to know the exact exchange rate.

If this wasn't the case you could just avoid capital gains by buying something with eulos and reselling it for usd with no profit.