In taxonomy for information management or web publishing, you are limited by the shape of the content. However granular your terms, most CMSs can only apply them to whole documents or pages. Some organizations have more complex content management needs.
Document categorization and indexing is often either purely manually rule-based or purely driven by statistics. In real-world scenarios the short comings of both approaches, respectively, prove problematic: Rule-based approaches require a lot of resources and insight.