We need to provide a way to save the same statement in multiple languages. Examples for its usage are : the motto of a country that has several primary languages, usage notes and comments (Property:P2315) that should be supplied with multiple languages and fallback.
Clearly yes, if we have two sets of monolingual strings in different languages, we should want to know which set a individual string belongs to.
Multilingual text should be used if:
- Texts in different languages are translation each other, and only one and any one is needs, like labels and descriptions (e.g. P2559)
- The text is naturely in different languages and only at most one per language, or if there're more than one per language, the value can easily be split to sets (e.g. P2275)
Multilingual text should not be used if:
- The text is in only one language (e.g. P1922)
- There may be more than one text in one language, and texts in different languages have no direct relationship each other (e.g. P1843)
Another possible use case: multiple sitelinks to the same site. For example, Wikidata may have a project page for postal codes in English, and someone wants a similar page in German. But the community does not want direct translation, because English page may want to cover the whole world, while the corresponding German page may only want to concentrate on the aspects specific to Germany.
In short, we end up with multiple pages about the same subject, just like Wikipedia, that should have "sitelinks", but all pages live on the same wiki, and all would like to share the same wikibase item.
P.S. (tangent) TBH, I am not a big fan of the language-based content division because languages do not map well to territories, but this may be the reality of that community, e.g. if postal codes are similar in Austria / Swiz / ...