The Phoenix Suns were bad at pretty much everything last year. This year, they should improve significantly in a lot of areas, including becoming much more competent on the defensive side of the ball. A big reason they project as a serviceable defense is their long arms.
On the perimeter, having a long wingspan means getting into more passing lanes. More deflections lead to more runouts and easy looks on the other end. It also makes it easier to recover when getting beat, enabling a player to poke a ball out from behind or a help defender to break up a pass.