It is obviously used for christenings.
A world famous baptistery can be found in Pisa (Italy), next to the church.
The first picture shows the baptistery Saint-Jean in Poitiers (Vienne 86), the second one the interior of the baptistery in Venasque (Vaucluse 84).
|Baptistère Saint-Jean - Poitiers (86)|
|Baptistère - Venasque (84)|