Fous come off the bench because Keita give the team a strong rim protecting presence. It changes the way teams play and they learn quickly not to try and shoot over him. He also gets the opening tip almost every time.
If you watch Traore play you will find that he is dominant in the post when the defender is 6'9 or shorter. When they get to 6'11 and taller he struggles. He is really only 6'6.
Coming off the bench is a change of pace in how BYU plays and they try to move the ball inside more.