When adding a template with no TemplateData, VisualEditor says "No unused fields". It would make more sense to say something like "No known fields", or to omit the message and/or the whole box entirely.
When adding a template with TemplateData (which could be incomplete), after you've used all of the known ones, VisualEditor says "No unused fields". It would make more sense to say "All known fields have been used".
The basic semantic point is that there may be fields (parameters) that have not been used, but that VisualEditor and TemplateData don't know about.