Why pastry should be boiled and why custard sauce should not be boiled?

Pastry is often boiled or blanched to set its structure and create a flaky texture, especially in preparations like pâte à choux, where steam is necessary for leavening. In contrast, custard sauce should not be boiled because high heat can cause the eggs to curdle, resulting in a grainy texture instead of the smooth, creamy consistency desired. Instead, custard should be gently heated to thicken, allowing the flavors to meld without compromising its integrity.