If you still wonder what the flying gold coins do, that`s not so bad, because it is really hard question. But one thing is for sure – they give Free FarmVille Cache… sometimes. Yes you can get FV Cache sometimes.
You can see a flying gold coin while you are plowing your farm I (only when you are plowing). But this is like a lottery. You can get the FarmVille Cache, but you really have to plow a lot. And this is the only free way to increase your FV cache value without paying money and keep your PayPal account and credit card full.
Use fast growing crops, than harvest and plow.
Plant blueberries or even raspberries.
Planting raspberries will not give you experience, but if your aim is FV Cache, sacrifice your XP. And you really have to be physical strong to do that. If you just want the easy way, use PayPal or your credit card.