Between scouring dungeons for loot, trading with merchants, and paying off shady businesspersons, money is a major component of many video games.
Maybe developers realized that greed is good and that the accumulation of wealth is hugely motivating. Or maybe it's a subtle reminder that capitalism dictates every moment of our lives away from the controller. For whatever reason, a currency counter is always at the top of our inventories.
Out-of-universe, this at least makes sense for some kind of point system. In-game, however, sometimes the cash flow doesn't make a whole lot of sense. Here are a few of the least functional video game economies and the currencies on which they are based.