Your way of thinking is so one dimension. Of course those have something to do with it.
Matic, Scott & Fred offer different things, we can rotate them depends on who's our opposition.
Against lower team, Matic will be much more suitable candidate to play alongside Pogba & Bruno, his composure and his ball playing on the ball is needed and he's a type of player who is suitable when playing in much more possession. While against team who is not afraid to attack, Scott or Fred will be much more suitable due to their work rate. Fred can also be used against the lower team since he's good on the ball and his awareness & composure have improved this season when under pressure.