This pastry was used in the Middle Ages for custard or cheese tarts. The pastry was hand raised and hot water is used because it softens the gluten and makes the pastry easier to handle and less prone to break. It also holds its shape nicely for baking. Later lard and butter were added to the recipe and it evolved to the hot-water pastry we know today.
The pastry is incredibly fun to handle; when baked it will appear tough but this is the traditional pastry for these tarts and pies.