A relation may refer to a higher level asset type, for example Scope.Workitems refers to Workitem assets. Downcasting allows the relation to be filtered to a more specific asset type and also the relation to be filtered by attributes of the more specific asset type.
Downcasting Scope.Workitems to Story assets allows you to use Story specific attributes in the filter such as the Category attribute.
For example, this URL query will return a 404 error:
While this URL query will work correctly: