Part of the art of Game Mastering is to create the alternate reality that you and your players inhabit during the course of the game... and it is often the little things that make that setting come vividly to life.
One such 'little thing' is describing the everyday coinage that the characters handle. To them, cash may be very important. Why else go adventuring, after all? You cannot live on fame alone. 'Heaps of gold coins' may sound nice in a treasure description but when you get down to the nitty-gritty of everyday life a 'gold piece' is pretty boring. Not to mention, does EVERY single community in your entire game world use the same currency?
It's much more fun if one town uses mesetas, say, while another won't touch anything but florins. Mathematically-inclined GMs may want to play around with exchange rates, but even if you decide that a 'gold piece' has the same value whatever it is called, it can provide greater realism if they have different names depending on where you are. It can even prove source of adventure: perhaps your characters are loaded down with gold taels that they cannot spend where they are on account of the state that issues taels being at war with wherever it is that they are now. Can they find a backstreet moneytrader, or will they be accused of being spies?
A useful little resource.