Back to first principles: what does Black attack? His rook attacks White’s rook on f2—which is guarded only by its king. Black can't drive the king away with a check, but he looks for any check he can give and does find one in Rxd3+. It loses the rook but draws the king away from the f2 rook, which Black takes a move later. Black wins a pawn. The point: a check sacrifice with a rook is most profitable when it enables you to take an enemy queen that had been protected by its king; but it still may enable you to turn a smaller profit when the king protects one of its rooks.