Because it wins material, as at the moment my king has no flight squares. I think other than taking it, the only other move for white is Qh1 to give the king a flight square on g2. Qh1 also has the advantage of forcing off the queens, so it may have been a better move. But while playing the game, I never considered Qh1 as I just thought "He is threatening a very nasty check on d4, I've got to stop that".