Why is boiling water essential for choux pastry?

Boiling water is essential for choux pastry because it helps create steam, which is crucial for the dough to rise and puff during baking. The high temperature of the water also helps to gelatinize the flour, ensuring a smooth and cohesive mixture. This process results in a light, airy texture that is characteristic of choux pastry, allowing it to hold fillings like cream or custard effectively.