When inserting certain templates in a translation (such as infoboxes), no fields are to be filled by the user. The template uses the available information on Wikidata and thus will work properly once the article is linked to the proper wikidata item. However, since the Wikidata links seems to be established after the article is published, those templates are not shown with their final aspect in translations in progress.
Allowing the template preview to use information available from Wikidata will allow the translator to detect mistakes earlier and improve the translation quality. For example, it will allow the translator to know which fields need to be translated and which ones should be left empty because they are best regulated by Wikidata.