They were found on the junkyard world of Raxus Prime and were held together by the Force. The Scrap Guardians served as the core component of Master Paratus's droid army on that planet. Formed from detritus and cast off parts, they were animated by Paratus's Force abilities and served as sentinels created to defend Kazdan against hordes of opportunistic scavengers.
They served as Kazdan's eyes and ears, alerting him to any enemy's approach well in advance. As soon as an intruder came near, the summoned Guardians would assemble and emerge the scrap heaps like zombies from the grave. They were well-suited to battling a Sith, being armed with blasters for ranged combat, though they also made use of bladed arms to attack their enemies in close quarter engagements. They also appeared to have the ability to disband their component parts, move their force essence to a new location, and then form a new body out of scrap parts at the new location. They used this ability to great advantage when Starkiller landed on Raxus prime by attacking him from afar and then moving to the new location and engaging him in melee combat.
Once they were formed, they served as an effective army against the Drexl Roosh's scavengers.
Darth Vader dispatched his Secret Apprentice to Raxus Prime to kill Kazdan Paratus. In his path were dozens of Scrap Guardians that attempted to defend their master. However, once Paratus was killed, the guardians collapsed into their junk components. Without these guardians, the planet was quickly conquered by the Galactic Empire.