Bookmarks in Docs are just like a bookmark that you would use to mark your place in a book. You use bookmarks to mark a location in a document so that you can quickly find and jump back to it. This is quite helpful, especially when you may have lots of documents to read/edit.
Another use would be to allow you or your readers to jump to a specific part in the doc itself. To do this:
Insert a bookmark at the specific part of your doc
Click on the bookmark
Right click where it says link
Copy the URL and place it in your document.
Embedded below is a 1 minute and 45 second YouTube tutorial.