Question

why do you suppose many different structures are needed by the digestive system

Answer 1

 The food that the system processes is made of different things (fats, carbs ,proteins, minerals, fiber, etc.) and it is in different forms (solid, liquid, mush, etc.) naturally you need different tools to process different materials.

Answer 2

There are various food that can not be digested just like that. So that is why the digest system needs different types of structures.