The first-level title is equivalent to "chapter", the second-level title is equivalent to "section", and the third-level title is "section" under "section", which means more and more detailed anyway.
1, let readers have a general understanding of the content and structure of the full text before reading this article, so that readers can decide whether to read it, whether to read it intensively or skim it.
2. It is convenient for readers to choose a sub-argument in the text. A long paper has many sub-arguments besides the central argument. When readers need to know more about a sub-argument, they can rely on the table of contents to save time.
The table of contents is usually placed in front of the text of the paper, so it is the guide map of the paper.