Dough is mixture of flour and water. The combination of these chemicals results in the formation of gluten.
gluten A tough, elastic protein. Gluten gives dough firmness but also allows the dough to stretch.
Dough usually contains one or more leaving agents (substances that produce gases in a baked product). During baking the gluten in dough is stretched by the carbon dioxide produced by the chemical reaction of a leaving agent such as yeast or baking powder. Once the gluten has been dried, its gives the food a firmness.
Fat in the dough cause the gluten strands to be broken and thus the gluten molecules are shortened. Because fat shortens the gluten molecules in dough, fats commonly used for baking are often called shortening. The shortened gluten molecules causes the dough to easily break, thus it is said to be flaky.
When I bake cornbread, I add lots of fat if I want the cornbread to be very crumbly and little to no fat if I want it to be firm.