Why do we have to know the historical background in the novels?

I believe we need to know the historical background of the novel that we choose to read because without knowing it we could totally miss what is happening or not understand why things happen in the novel. If we don’t understand the situation or place the novel is put in then the reader could be extremely confused to why things are the way they are. If a book is about a story that happened in the Holocaust that book shouldn’t have to explain what it is, it’s something the reader should expect to know, if they don’t there isn’t a reason to read the etory.