They are divided in several subcategories, that you can select and browse in the building menu.
The main purpose of decoration buildings is to boost the supply of needs from business buildings in properties where they are placed.
Applying upgrade tokens to decoration cards costs money and it increases the stats: wealth, and the power of the boost they supply to businesses.
Decoration buildings can freely be moved around, or even removed, depending on what you want to achieve, it's entirely your choice.