Why do I have to login w. Google Drive on mobile, after all?

I never quite understood it, because whether I log into it or not, I either have to login using my Battle.net or the associated google account once more.

Btw, their "Login with GDrive bc hearthstone will check whether there is any progress stored there" message never made any sense to me, simply bc hearthstone doesn't store anything at all on your GDrive (I checked), so what's the point after all, waste everyone's time?