Quite simply, we want Keita to be the controller in our matches. So under Rafa, this was the role of Xabi.
He isn't dropping in the CB as much to help them move the ball forward, he is providing an option
Once the ball is in the final though, what he does more resembles Xabi in that he stays in support and doesn't get ahead of the ball too
Where he differs though, is he also performs the role as a ball carrier - which neither Xavi
Which means whoever is partnered with him needs the tactical intelligence to know when they should drop in for Keita
The reason this is a problem for our fanbase, is they have been sold on him being a weird mix of 'Ribery + Kante'. Or 'Deco + Makelele'.