World-building should never be done inside your book. This is another of Corey’s writing rules. Note: Corey’s first writing rule is you can (and should) break every writing rule if a) you’re good enough to pull it off, b) do it well enough to get away with it, and/or c) break it in a way that hasn’t been done before.
So, that’s why I like this world-building system. A quick and dirty, index-card based system for generating just the most important facets of your world. Once you’ve come up with them, they might find their way into your story organically, or they might not, but anything is better than paragraphs or pages, or even chapters, full of expository world-building that you couldn’t bring yourself to cut because it shows just how damn inventive you are.