What is the purpose of the villi and microvilli of the small intestine and the circular folds of the large intestine?

The villi can increase the surface area in the small intestine for absorption of digested food. As the villi has a thin wall, one cell thick, which can speed up diffusion, and a network of blood capillaries to absorb and transfer nutrients.

And in large intestine, those folds can also increase the surface area for reabsorption of unabsorbed food, mostly water.