Digestion of food, is only half the job. The other half is gettingthose molecules from inside the intestine to the blood to be carried to cells throughout the body where they will be used. The structure of the small intestine, as explained in the following video, plays the key role.
Nutrients move from the intestine into blood by simple diffusion, which is a relatively slow process. This slowness is overcome by vastly increasing the inner surface area of the intestine that is in close contact to blood capillaries. (See: intestinal villi explained) (Please post questions.)