To guarantee that no team can meet the same group winner who they have already played.
The winners from groups A,B,C and D play the runners up from E,F,G and H
The winners from E,F,G and H can play the runners up from A,B,C and D
Otherwise there would have been a chance of the last two teams in the draw having played in the same group.....
50% chance of one of my (and Horsham's) preferences then...