For accounting it's better to give things an expiring date. Perhaps 5 year would have been better than 1 to keep people happy, but I don't know why they decided 1 year (or whatever is the exact number).
This is a complete nonsense given I have video games with virtual currencies in them that I've purchased for real money and the currency is still sitting there well over a decade later.
I guess in the game, you already bought the fake-gold-coins and you can "enjoy" having them even before exchanging them for fake-bread or fake-swords or fake-whatever.