Why Were Gargoyles Put On Buildings?

Gargoyles actually began during the middle ages, when they were a means of adding drainage spouts to stone buildings.  Water would run down the rain gutters from the roof to these spouts and then be directed outwards and away from the building to stop rain penetration.  Moreover, the gargoyles stuck out from the building so that the water did not erode the soil.
Gargoyles started off life as just plain pieces of stone, but then began to be carved into elaborate architectural features.  It was normal practice for the rain which ran into the gargoyle to be drained out from its mouth.
As time wore on, gargoyles began to be associated with frightening away evil spirits, which is why many of them are particularly ugly and menacing.
From the 1700's when lead drain pipes were introduced the gargoyle effectively became redundant, but was sometimes used as a feature and to display the building owner's wealth.