Special:GoToLinkedPage is a great tool, but It needs wildcard support, when the specific Wikipedia article language is not known. For example, If I view an OpenStreetMap object in France, which has a WD id, I would like to open a corresponding Wikipedia page. Knowing the user's language preferences, e.g. German, I could first specify it as site=dewiki. As a fallback, I could specify the language of the land: site=dewiki,frwiki. But if neither languages exist, I would get a "this combination does not exist" error, which is obviously not very helpful.
I could append a long list of all known languages, and all existing Wikipedia projects, e.g. wikivoyage, but sending a giant string with every GET request seems bad. Or I could simply say site=dewiki,frwiki,*, and let Wikidata pick one for me, using either some magical decision making process, or even random.
Why? Because showing a Wikipedia page in ANY language is far better - I can use Google Translate to understand what it is about. Showing an error message is like playing a guessing game - nope, you got the wrong combination, try again.