They can then provide width on both sides at the same time.
The midfield unit of five in a 3-5-2 formation provides both width to build around the opposition and numbers in central midfield to play through them, without the need for much in the way of movement or rotations. The three central midfielders can draw opponents into the middle of the pitch, before play is then spread wide to the wing-backs. They can then provide width on both sides at the same time.