Because we play significantly better with a double pivot rather than a midfield two. It helps us look much more solid defensively whilst giving Bruno a little bit more freedom to play risky passes.
With that extra defensive help, the backline is finally going to finally to get some legs in front of them. Maguire has made a few individual mistakes in recent weeks, but they've been overblown due to it being a handy way to keep the narrative of the Greece incident rolling in the press. He needs to play himself back into form and into confidence. With McTominay and Fred in the side, Ole's selection gives him the opportunity for both.
Offensively, I would have preferred Greenwood to Mata. But Mata has really good chemistry with Bruno, so there's a positive there. James gets a tough rap on this forum. He's not a world beater, but he's a capable threat on the break and his pressing game is very good.