If you're a fan, geek, otaku, what have you there's a good chance you're into SOMETHING that has a well defined world. It could be the World of Warcraft, it could be the Trekverse, Middle-Earth, what have you. You could have several properties you like.
You may even work on them. Perhaps you write fiction, make MMO's, etc. You may be crafting the setting for a multi-part RPG right now, or designing the starships for a futuristic combat game with a detailed timeline.
Worldbuilding is very important to us geeks, and even more important to the fictions and worlds we build.
Ever wonder why?