If your document has many headings, it's a good idea to add a table of contents in case it will be accessed outside Google Docs, for instance, if you export it as a PDF or EPUB document. You can even ...

Google Docs is more powerful than it appears. You can create long-form projects on Google Docs as well as a table of contents.

You can make a table of contents in a Google Doc with a computer or iPhone, and create organized navigable sections for readers.