Think of it sort of like this:
3 at the back vs 4-2-3-1
Ranocchia Samuel(Milito) JJ
(Eto'o) Cambiasso(Sneijder) (Pandev)
You leave Milito 1 v 1 with Samuel - a situation he's going to score at least 1 goal a game from.
The traditional 4-4-2 approach would be more like
Zanetti Ranocchia (Milito) Samuel Nagatomo
(Eto'o) Cambiasso (Sneijder) (Pandev)
Which means you're doubled up on the CB at all times - Samuel can deal with any through balls from Pandev, vice versa Rano/Eto'o, and Cambiasso cuts off the direct supply.
And of course if you move away from playing an attacking trident, to, say, someone using a flat 4-4-2, one of the wingers pushes up (so, say, Eto'o and Milito up front, Stankovic and Pandev on the wings). Play is attacking on the left wing.
(Pandev) Zanetti Ranocchia(Milito) Samuel(Eto'o) Nagatomo
Cambiasso (Stankovic)
So your full back on the opposing wing fills in at the "corner" of the box, thus meaning he can cover play if it is switched to the other flank, whilst offering a 3-v-2 situation in the box, and both central defenders are ball-side of their man, with Zanetti there to block the cross.
edit;
I cant align Stankovic properly, he should be roughly infront of Nagatomo