He’s probably there because he was involved in that scheme to assign 100 DOJ attorneys to review Kavanaugh’s papers and might have been called to answer for some DOJ opinions on various topics (either called officially as a witness or on the side). Either way I’d suppose the #2 or #3 person at the DOJ would have at least a professional if not personal interest in the SC nominee hearings.
That’s as good as any explanation I’ve heard.