Generally we don't really consciously do it as a purpose-driven thing. . . . It's really just a response to life.
Once you get into the era of the printed book, it gets a little easier. After years and years, you make a serious survey of that literature, and then you make it more specific depending on what kind of case appears. I never set out to collect material on cheating at bowling, but I found out that after 20 years, I had a lot of material on cheating at bowling.