I have never done that.... never seen it in a concert either. The only place I've ever seen it was in some churches where they don't want the drummer drowning out the guitar players and audience.
So I really have no idea what it would be for. I'll take a wild guess. I don't see any individual mic's on the drum kit.... that doesn't mean there aren't any but I can't see one. So I would assume that one mic catches the entire drum kit as the sound is diffused by the shield.
They probably aren't using mains.... that is, running all instruments thru main speakers - - so this "may", (remember, I'm only guessing), give the sound engineer control to balance the drums with the rest of the musicians.
