Well the answers to that is pretty obvious.
1)Gundogan offers much better passing from the deep position. I love Fernandinho but there's been quite a few times when we've not been able to capitalise on a break or make an effective swith of play to put a winger in because his passing isn't quite good enough for that position. His aerial passes in particular are quite loopy and not that accurate.
2) It would allow us to play Silva, Gundogan and De Bruyne. Gundogan was Pep's #1 target for the summer, you can expect him to be in the team when fully fit, and the other two are performing brilliantly at the moment.
3) I absolutely love Zabaleta and Sagna, but RB is IMO our weakest position in the starting XI and we'd be strengthening it - and the current fullback role of drifting into the centre of the park lends itself perfectly to Fernandinho's abilities and would beef up a technically brilliant but slightly lightweight midfield 3.
I'm not sure if it will happen, and if it does I don't expect it will be that often, but the reasons for doing it are pretty obvious really.