The benefit is directly towards WMDE's goals.
Specifically,
Goal 1b: Grow the reach of Wikidata beyond the Wikimedia projects
A Wikidata property = Some external vocabulary property
A Wikidata property is similar to Some external vocabulary property
A Wikidata property is considered a parent class to Some external vocabulary class <-- Needs new property for support.
A Wikidata property is considered a child class to Some external vocabulary class
What Schema.org needs now from Wikidata is the addition of a few new properties that seem to be missing. (I sound like a broken record now)
1. 'external subclass'.
DONE.
Let's start there. I don't even want to drag this discussion further than beyond that 1 request, at this point in time.