From the website:
A strong core of reliable and adaptable warriors that can lay down fire while advancing or holding down terrain is an invaluable aid to any tactician. In the case of strike forces bolstered by Primaris battle-brothers, this role is fulfilled by the Intercessor Squads.
All Space Marines are experts of the rapid strike, but Reivers take these skills to the next level. They are specially trained and equipped to launch brutally effective and unexpected assaults. They do this with a sudden fury and shocking violence, for the Reivers fulfil the role of terror troops that sow fear and confusion amongst their foes.
Shambling across the battlefield in reeking hordes, Poxwalkers engulf their enemies in rotting tides of grasping hands, gnashing teeth and squirming tentacles. They are the cursed victims of Nurgle’s plagues, transformed into unliving weapons by the cruel masters of the Death Guard.
No Traitor Legion stands as high in Nurgle’s favour as the Death Guard. Swollen with the Plague God’s blessings and utterly devoted to the corruption of realspace in his name, the hulking Plague Marines that comprise their mainstay despoil all in their path while chanting praise to their gruesome patron.