Many people are scratching their heads, and wondering why we need another CB, after Kompany and Mangala have started so well? You only need to look at last seasons performances, and a history of injuries for Vinny, he's not getting any younger and we need our captain to hopefully last the majority of the season without incident, which means not playing him in every game, if we are in all the Cups until later stages we may find fatigue and niggly injuries on the increase, we might have to treat him the same way we do Aguero.
Then again he might pick up an injury which keeps him out for weeks or months, so having a CB who has played with Mangala already looks like a perfect back up plan.
Another thing is, players respond to competition for their places, it might keep all players on their toes if there's strong competition throughout the squad,