Putting Giggs in the middle solves a few issues...
1) with him there, instead of just Ronaldo, Park, there is
another player who can bring the ball out of defence and launches
counter..
2) his leadership is crucial in stability.. With GaryN, both of
them are stablising the ship.
3) He is able to link well with the forwards in that, he likes
to release the balls to them to score instead of holding them like
a Nani.
4) lastly, but importantly, set pieces. I just loathe to see
Rooney, Park or Carrick taking corners, inconsistency sets in. With
Giggs, his deliveries are consistent and Vidic/Ronaldo can look
forward to good crosses.
Just my two cents.