Why the topics are important for reading?
In my own opinion, we really need to have an specific topic. If we are at the bookshop and have no ideas to buy a good book we can choose a book from the interesting topics because topics can help readers to have a direct understading... It may give readers a sense of direction on to what extent . Topics are things that the author discussed in many parts of the story and they are the core of the story that the author want to convey to the readers.
It is really indispensable to have a right topic, so you will have a full scope and understanding right knowledge and information you want to attain