What is and what is useful for?
A faceted navigation is a navigation system which offers multiple facets (aspects) of navigation to a single piece of information. This is done by using FacetedClassificationInMoin to group bits and pieces of information together (more in Faceted_navigation).
In MoinMoin, faceted navigation would expand the classical concept of Category, making it possible to:
- Help the users to find what their are searching, because it make more (ordered) paths to the same piece of information.
Creating AutomagicalMenus from the facets.
Interested? 
See:
FacetedClassificationInMoin for a background on the idea,
PagesHierarchy to see from where it sparked,
FacetsEngine for some discussions about implementation,
FacetBrowsingTheme for a discussion related to the user interface.
Other implementation
Another (possibly useful/related) implementation in Python is DiamondWiki.