Worldbuilding in some sense is a requirement for all writers. The people and places in fiction may have analogs in real life, but a writer in the U.S. cannot depend on every reader (or even most readers) being familiar with the Lincoln Park area of Chicago or the lower east side of Manhattan, much less the streets of Bombay or London or Ladysmith. The writer therefore has to recreate the real place in her fiction, choosing key details that evoke or imply a raft of other things that add up to that particular place and culture.
For those of us who write fantasy and science fiction, worldbuilding is even more of a necessity. The places our stories occur often have no real-life analogs; one cannot travel to Edoras or Cair Paravel to check out the sights and sounds and smells. One cannot look up the fashions of the Galactic Empire or the social customs of the kzinti or Klingons. The writer makes them up.
One of the first things you find out when you start paying serious attention to this is that every detail you invent implies other things, large and small. A codfish dinner served in a town far inland implies not only a fishing industry, but fast and reliable transportation (or the fish would spoil before they got to the table). The existance of such fast and reliable transportation means news will move as quickly as the fish do, so if you want it to be three weeks before they find out about the magical thunderstorm on the south coast, you suddenly need to come up with a really good reason why they wouldn’t hear about it a day later like everyone else. And so on.
Back when I was still getting the hang of all this, I discovered that one of my biggest problems with making forward progress was that I’d forgotten to make up some aspect of my imaginary world that I suddenly needed. The heroine arrived in a new town, and I’d forgotten to make up the architecture; the city guard showed up and I had no idea how they worked; a foreign diplomat arrived and I had no idea what he considered a proper, respectful greeting and what he considered an insult.
So I started keeping track. Fast-forward ten years or so. I had a twenty-plus-page list of things to think about, and it was still growing. I mentioned this on the Fidonet echo I was on, and people talked me into posting the list. One thing led to another, and my fantasy worldbuilding questions have been up on the Science Fiction and Fantasy Writers’ of America web site for … I think it’s getting on for fifteen years now.
Every so often, I get complaints about them. Interestingly, the complaints are always that I left something out, not that X or Y is not really important to worldbuilding. I always tell the complainers the same thing: The fantasy worldbuilding questions are my list of things I have a tendency to forget to think about. Stuff that I always remember to think about is not on my list. If they forget different things, they should make their own list of reminders.
But people persist in trying to make the questions into a prescription or a recipe. And of course, once again, there is no one recipe or set of rules that work for this aspect of writing, any more than any other. I know quite a few writers who do little or no worldbuilding in advance – they have the sort of brain that needs to not be tied down to a previous decision (and they also seem to have a gift for making everything tie together, even if it was made up on the fly).