Why does the Notre Dame have Gargoyles?

Aside from their function to ward off evil spirits, the gargolyes had an architectural function. They were waterspouts, projecting from the roof gutter or upper part of a building to throw water clear of walls or foundations.In this way they protect the mortar and the stones of the building from erosion. Gargoyles are widespread on medieval buildings. That is because dividing the flow of water minimalized the potential damage coming from each gargoyle's mouth, and because of the great number of gutters carried on the top of flying buttresses and walls There is a v. interesting article about gargoyles at http://www.ulrikehoinkis.de/gargoyles/gargoyles.html

Its to scare off bad spirits and evil spirits from the place same with dragons on places.

Gargoyles are not devils, they are on the building to scare away evil. The one thing you will also find many of them are repeated in buildings in the same area. Like you may find two churches with in blocks of each other and they may have one or two that look alike. Normally it is because one is easier to carve that the others. I saw a show on TV one time about all the different kinds there are , very interesting and many of them have been done by famous artists.