Look on your bookshelf (or your Kindle). What do you see? You see titles (and authors). For most non-fiction Fiction is different: the title is more of a hook than a topic. books the title gives you a fairly good idea of what you will find in the book. Apart from helping you decide whether to open the book, once you do the title prepares your mind for understanding the contentThis is known as schema theory.. The title gives you the main idea of the book. The book itself is almost always divided into chapters and the chapters themselves have titles so that you know what the main idea of each chapter is. So there is a pyramid, hierarchical structure which is helpful in non-fiction texts.
This same pyramid structure is found at a lower level in the text itself: each paragraph usually has a topic sentence, which is the controlling, main idea for the whole of that paragraph. The rest of the paragraph is detail about this main idea. The detail may be explanation, supporting arguments, evidence, reasons, and so on.
This method of text organisation is helpful for the reader because it helps with understanding the content of each part of the text, and it shows, or hints at, where the text is leading. It's also very helpful to you, the writer, because by following this pattern it helps you to write well organised, coherent texts.
The example below shows how a topic sentence is used in an expository text and how cohesive ties in the paragraph point back to the main idea.