The traditional deployment of a 4-3-3 calls for a center forward and two wing forwards, under Leo Milan operated this way with Ronaldinho, Borrielo and Pato. Allegri, like anyone else with two eyes, sees the lack of balance in such a formation and deploys the strikers in a much more compact triangle calling on width from his fullbacks, while still shielding the midfield to some extent. This deployment doesn’t unbalance the formation in the midfield, but shields it, simply because of the amount of player traffic in front of the opposing central defenders and opposing defensive midfielder. While none of the strikers may actively defend, their mere presence in that part of the pitch makes it difficult for opponents to move the ball into the midfield. It is also a challenge for opponents to mark the strikers, which is the true beauty of this deployment.
Источник: The Offside