The game doesn't "need" it, but the idea was to make the time management game more interesting by adding an economic management element to it. So now since the price of 'gold' in the game can fluctuate based on the exchange rate to fiat currency, it can change how you play. Stock up on gold when it is cheap, maybe sell off items like a level 10 archer when it is high. It provides an additional incentive to use the free market trading system in the game. You can trade anything you want, you can trade a village for an army for example, or sell a Skirmisher, or whatever. You and the other party negotiate it entirely between yourselves. If you are done with the game at some point and still have money in it, you can essentially cash it out on a public exchange.

Our team has worked with time management games for years and while the code on this is tight, fast and modular (we have the same code for ios, android, windows, mac and linux and the server code is GO), that isn't really visible to the players, so this was something we thought was interesting.

thanks for your question