Here's why authors might use this structure:
* Clarity: The sub-title often provides a more specific explanation of the book's content or theme. In Vonnegut's case, "The Children's Crusade" adds a layer of meaning to the story's exploration of war and its effects.
* Emphasis: The sub-title can highlight a particular aspect of the story that the author wants to emphasize.
* Intrigue: The "or" format can create a sense of curiosity and make the reader want to know more about the book's content.
The use of "or" in a title is not always necessary, but it can be a useful tool for authors to convey more information about their work.
It's important to note that not all books or movies with "or" in the title have the same structure. Sometimes, "or" is used to indicate two separate titles for the same work, or even to indicate a different title for a different edition.
For example, a movie might have a different title for international release. It's best to look at the context of the title to understand its meaning.