Why do authors include violence in their texts and for what purpose(s)?
I think that authors have been writing about violence for as long as mankind has been able to write, and I'm willing to bet that even before we could write, we were telling one another stories that involved violence. The mythologies of most cultures are filled with violence, and I would hazard a guess that ancient hieroglyphics tell tales of violence. The Bible is still another example of a violent literary text. Since this seems to be universal, I think it is fair to infer there are reasons for violence in literature, that it meets the needs of both author and reader.
To the degree that we expect literature to reflect life, it would be impossible to make literature violence-free. The world is and always has been a violent place, and we see examples of this daily. Whether we are reading fiction or non-fiction, it would be remarkable not to see some violence. If all literature were violence-free, it could not act as a clear window for us to peer out of or as a mirror that allows us to reflect on ourselves.
Violence is a form of conflict, which is a requirement for almost any story. We are not inclined to read a narrative that does not include some sort of conflict and resolution. Not all conflict is violent, certainly, but violence is part of the author's repertoire of conflict plot elements. If we eliminate violence as a plot element, we are left without some of the greatest stories in literature: A Tale of Two Cities and War and Peace come to mind. And there are thousands more.
For an author, beyond being a plot element, violence can provide a lesson to be learned by the reader or be the author's way of working out his or her own feelings and thoughts about violence. The Things They Carried is a good example of the former, and Night is a good example of the latter. Writing this story must have been cathartic to Wiesel, I would think.
Authors also include violence because it meets the needs of the reader. We are attracted by tales of violence, perhaps because we can tell ourselves it will not happen to us, perhaps because reading about violence is a means of satisfying our own desire to commit some violent act, possibly because we want to find a lesson to be learned, or perhaps because violent stories are in fact entertaining. We enjoy sitting by the fire reading a scary story, one filled with blood and guts, or at least, most of us do. We are safe, we are warm, and we are thoroughly engaged.
There are authors, I'm sure, who add violence as a way of selling more books, but I would say that most of its inclusion is purposeful, to meet the needs of the author and the needs of the audience. If the violence is gratuitous, not making a point or advancing a plot in some way, I find it disturbing, but I find the alternative, somehow "cleansing" literary texts of violence, even more disturbing. If we were to do that, we would not be terribly far away from Fahrenheit 451, with someone above us, judging what is or is not good for us. Let us deal with the violence we read about, and let it teach us, act as a substitute for our own violence, or even entertain us.