Controlling the flow of money is an important way of balancing your campaign. Too much—or two little—money can ruin the fun of your game. Give your characters mountains of gold and game is spoiled. Suddenly wealthy, they no longer have the urgent need to adventure that impending poverty can provide. Too often they can buy their way out of difficult situations through bribery or "throwing money at the problem."
Worse still, they attempt to apply modern, capitalist ideas to a quasi-medieval world. They may try to hire an enormous staff of wizards to mass produce potions and scrolls. They may set up shops to make assembly-line armor. Advances in organization and production like these come slowly over time, not all at once. You may have to remind your players to limit themselves to the knowledge and attitudes of the times.
It is equally bad to keep your characters too poor. You are creating a game world for a fantasy role-playing game. If the characters are so poor that they must count every penny they spend, they are leading squalid and unhappy lives. Reward them when they accomplish things. You shouldn't always frustrate their desire to get rich. It's just that wealth should come slowly, matched to the level of the character.
Even before you play the first session in your campaign, you can use money as a tool in creating your game world. The form and shape money takes is by no means standardized. The simple monetary system given in the Player's Handbook is just that—a simplified system for coinage. It is not absolutely true to the real, historical world and is not even an accurate reflection of most fantasy worlds you find in books. It's just one way to approach money.