When developing a world, whether it is in a secondary world fantasy (epic) or modern fantasy (urban), or even Science Fiction, you must create a completely believable world. If you have magic, there must be a system to it, there must be rules, and you cannot break those rules. The same goes for Science Fiction. If there is space travel, there are rules you must set and stick to if you are to create a believable world. Technology or Magic, when creating a story, World-Building is important.