Bonus buildings are ornamental; they don't do anything but look nice at the cost of maintenance. Each bonus building can be bought in your Profile menu with 200 gems. Random bonus buildings can also be created by Ibn Al Hakim at the Academy of Wisdom neutral power.
Ornamental buildings attract citizens that do different things in their presence. For example the cemetery a under keeper comes out and digs followed by your citizens to pray for the dead. With the pillory the person gets dunked while a crowd cheers. The juggler camp has a dancing bear, belly dancer, juggler, and strongman lifting weights. If you want an area to look very regal, build heaps of Flag poles and Pillorys.
The Venice add-on also includes new bonus buildings. But they have not been added to this wiki yet. They cost 300 Gems instead of the normal 200.