Decorations are needed on every farm to keep your Workers happy.
The happier you can keep them, the lower your running costs will be, so you will want lots of decorations with high Happiness points which appear in green after the smiley icon. Many decorations are available under the Build menu for Farm Cash, but those with the highest Happiness cost Gold. As your level increases and you need higher levels of Happiness, you will want to store or demolish old decorations - they cannot be used on your other farms. Demolishing reimburses some Farm Cash, but Gold is not refundable.