Many authors have blogs where they promote their books. This works particularly well for non-fiction, where you can easily blog about the same topics you wrote about in your book(s), bringing in search engine traffic and visitors from social media.
Fiction authors also use blogging, often as a way to stay in touch with readers, to talk about their writing process, and to share their own lists of favorite books in their genre.
Some bloggers do this the other way around: they start a popular blog on a specific topic then write a book about it. James Clear is a great example of this: he was blogging about habits before he wrote his bestselling book Atomic Habits.