In any book, the author must introduce the characters, the situation, and the basic elements of the setting, but in fantasy and science fiction you might have a viewpoint character in the first chapter who isn't even human, living on a planet that isn't even in this universe. The very laws of physics might be different, never mind the differences in culture, history, civilization, and everything that goes along with that: systems of measurement, idioms, naming conventions, philosophical principles, mathematics, science...speculative fiction strives to introduce and immerse a reader in a world that might be completely different from that with which we are familiar.
IGC Publishing 