This tag was mentioned on two wiki pages (not by me). Also, taginfo had reported 100+ cases of its usage before I committed more. In general, it looked like another effort of connecting OSM and Wikidata which is usually considered a useful thing. It wasn't me who started the effort, but my knowledge of Overpass and JOSM was enough to mostly finish it.

I agree that royal_cypher:wikidata tag is not of high importance. That said, consider it as metadata like check_date:* which helps people and tools to process and maintain OSM data.