TIL, I think I've landed on these pages a decent number of times but never from wikipedia's internal nav. I assumed they were more of an adhoc occasional thing, not a standard for sorting all pages.
The little arrows next to the subcategories can be clicked to open up trees, so you have hierarchical data in there as well. Try click open eg Classes of Computers (With 41 direct subcategories, and 91 pages directly in the top level category, that's a big tree!)
