With David Johnson going down last season the Arizona Cardinals had to improvise and throw a bunch of different guys into the running back rotation. Chase Edmonds was one of those guys who helped fill that whole at running back. Although David Johnson is fully healthy and ready to go this season you still will see this young running back on the field.
The NFL is a copy cat league, for a while, it was super pass oriented and mostly only one running back usage schemes and offenses. Things have changed even with passing numbers being higher than they ever have teams have been using two running back systems more and more to not put too much mileage on these guys going later in the year.