Why do we need [[Category:Mathematics]] and
[[Category:Biology]] when we
already have [[List of xy topics]]?
1) Adding metadata to the article it belongs to makes it easier to find
and update. Having it elsewhere would in fact add aditional workload and
require additional learning, because to verify whether a categorization is
correct, people would now also have to check all the [[List of xy
articles]] page where it could be or has been entered. With a Category:
approach, they only need to keep an eye on a single page (the article),
and the categories themselves are structured through subcategories and
therefore easy to pick.
2) Category pages are not articles. Like talk pages and meta pages, they
should be logically separated from articles, which has numerous benefits
(easier searching/filtering, counting etc.)
3) In terms of information organization, your approach leads to bloated,
big pages (the list of lists is going to be hundreds of pages long, the
lists themselves are going to be 30K and more), whereas the category
approach generates all long pages automatically from the information
stored inside the articles. It is much easier to handle.