@witty - To cover all possibilities. Depending on what black does, the queen will need to either go to a7, b8, or c7. If it goes on b8, it will need to be able to move to h8, so the king can't block with c8. Go through all the branches in the solution and you will see what I mean.
Like I've said before, I wish every once in a while I would get a position like this and actually find the solution otb without a lot of thinking involved.