Every text has a structure. It is not just a random collection of sentences. The parts that make up the text are related in a meaningful way to each other. Recognising the way in which a text has been organised will help you to understand it better. The writer may, for example, be explaining two opposing points of view, or describing why something happens. Undestanding the text organisation will help you understand what the writer is trying to do.
Try these exercises: Exercise 1, Exercise 2, Exercise 3, Exercise 4, Exercise 5, Exercise 6