#Wikimedia categories have interlanguage links. Obviously the content of all these linked categories do not have the same content. Someone has to add the articles, sometimes it gets done and sometimes it doesn't. Often articles just do not exist.
When the facts that are implicit in what a category is about make it to all the items in all the categories, typically you have a superset in Wikidata. It does not stop there; items in Wikidata may be included that are not in any of those linked categories.
This is all theoretical unless ... unless you can query Wikidata and use the results. Much data has been added to Wikidata based on the content of categories and queries have been used to identify missing items this is done using AutoList2. This is one application; it is used by some of the "advanced" users of Wikidata.
What is even more interesting is showing what Wikidata things should be in a category. This is done using Reasonator. At this time for over 690 categories statements are included that define a query. This query is already complex enough that the Wikidata functionality will not be able to express the results..
These queries could be of use to "advanced" Wikipedians because it is a basis for identifying articles that have not been categorised or articles that still need to be written in their Wikipedia. For everyone else it is just interesting; this information exists and it is readily available. It is one way of learning that Wikidata knows for instance about 121,922 politicians.