How do we store information? On a physical library we store books on shelves, and index their position in catalogues. Since the beginning of the web we started building websites like we were used to build libraries, and at the beginning search engines were catalogs of websites.
But digital information is different: it allows content to be reorganized dynamically, and we don't need categories anymore, because we already have heaps of
keywords in each document, that can be searched.
And another document that is very interesting to read about the topic is: Ontology is Overrated: Categories, Links, and Tags