Why do people place ceramic owls on top of buildings?

Owls are one of the most feared species amongst the family of birds. Other birds flocks and poops on the building roofs and walls which are extremely annoying and makes the walls messy. It costs money to maintain the walls and it is impossible to clean it every time. Whenever the flocking birds see the silhouette of an owl they tend to stay away from it to avoid from being the next meal of the predator (owl). This method may not completely avoid the poof of the all birds but surely minimizes a great deal.