As mentioned, the more aesthetic products can be bought using farm cash. Over time and as the game developers continue to expand the game, there may be a point where you can gain monetary balances back from the game by selling on your farm to other people. This is just an idea, though. For the user at present, it allows them to make the farm more personal and different to other farms in your social network.
For more on this gallery, you can find the corresponding article on the iGeneration blog