Everyone has strong feelings and convictions that they want to express. And it feels good to let them out. An author does that when he or she writes a book.
I had strong feelings and resentments about the abuse I suffered when I was a child in an East Indian culture. I felt angry when I saw that Westerners were lied to about that culture, and told that it was one of wisdom, love and benevolence. I had to tell the world the truth, and did so in Chapter 5 of my book "Sweet Dreams and Terror Cells." And it felt good, cathartic. That is the best thing about being an author: to release pent-up feelings.