Wikidata is to include information to be used in articles of Wikipedia. One template in particular provides a great example of this ability; it is the "Authority control" template. The rather technical information that it provides points to external sources where you will find information about the same item.
All these external sources have their own purpose and in essence, you may find additional information and hooks to functionality about all kinds of everything. It is for instance well possible to inform you about the availability of a book in *YOUR* library in several countries like the Netherlands.
Authority control is not restricted to the English Wikipedia, as you can imagine, it is called different in many other languages; Normdaten is for instance the name in German. All these templates face the challenge how they keep up with all the new external sources that are added all the time at Wikidata. The English version only supports some 12 sources while Reasonator knows about more than 200 external sources.
For all these sources statements are added all the time and new sources are added on an almost weekly basis. This makes it obvious why it is best to choose for Wikidata when external sources are to be added in an article. The template makes it easy; it shows everything from Wikidata what it currently supports.