Four. That's how many competent centre backs Juventus have on their roster. Going into last season, and even at the winter break, tifosi around the world were calling for a centre back in addition to a striker to sure up the squad. Something changed, possibly a decline in Asamoah's performances at LWB, but Ogbonna is now being called a luxury signing. While I will admit, he isn't a dire need, his presence, or someone of his qualities, will be necessary for Juve to take the next step in competing on multiple fronts.
While Giorgio Chiellini is a world class option at Centre back, he is anything but that at left back. His recent performance there for the Italian national team during their friendly with the Czeh Republic is a testament to that. He was caught out of position a few times and his forward runs, while energetic, were clumsy and left much to be desired. It is no surprise that he was subsequently moved back to the centre for the opening of the Confederations Cup. Ogbonna can play the left back role much better than Chiellini, thus enabling Juventus to more easily switch to a four-man back line.
This versatility gives Juve the ability to be more unpredictable in their formation choice, and make it harder for opposing squads to prepare for them. Ogbonna would be vastly superior at left back to any of the options this past season, whether it would have been Chiellini, Peluso, or De Ceglie. Being left footed, Ogbonna is much more suited to the role, and is also much more adept at moving forward than Chiellini, while much better defensively than either Peluso or De Ceglie.
In addition to the versatility added by Ogbonna, something has certainly changed since the winter break, and while Asamoah's decline in form over the second half of the season is partially the reason, there should be a bigger worry. 4. That is nowhere near the number of competent centre backs a team who wants to win the Champions League should have, especially when that team regularly starts 3. Lets look deeper into this, as good as Barzagli has been recently, it has to be assumed that he will start losing a step, as at 32 he is not getting any younger. Leonardo Bonucci enjoyed what was easily his best season, and can be reliably counted on, and if he can stay healthy Martin Caceres is as good a fourth option as can be desired.
Giorgio Chiellini however, has to be considered an injury liability due to the number of matches missed over the past 18 months. Going back to just January and February, no Juve fan has good memories of the left side of the teams defense when both Chiellini and Marchisio were out hurt. The left side of Pescara's defense could have performed better! (Ok not really, but you see my point.) If Chiellini's injury had been timed differently, or if Juve had drawn someone better than Celtic for the knockout phase, they might not have progressed. Ogbonna will provide the insurance that gives the team much needed depth, not only in case of injury, but simply for rotation to prevent over-working the current back-three. That alone could prevent injury.
So, unless Juventini are content to settle for the Quarter-finals of the Champions League, Semi-finals of the Coppa Italia, and the Scudetto, then Ogbonna is a very important addition to the squad, and another (I'm out of fingers to count them on) shrewd move by Marotta (If the rumours of 10m and a player are correct).