The armor consists of a breast plate, belt, wristbands, Transformer helmet with faceplate, an arm shield, and pistol-style laser weapon. All pieces of the armor are visibly marked with the Decepticon symbol. The shield also bears the likeness of an unknown Deception warrior. (Whether this face represents some autonomous Decepticon intelligence existing within the Decepto-Pack is unknown.)
It is assumed that a human or group of humans outfited this way would be able to aid the Decepticons in combat with the Autobots. It is possible that the Decepto-Packs were created to counter the Autobots' aggressive S.T.A.R.S. program. It is unknown if the bearers of the packs would have been mind-controlled conscripts or voluntary converts.
Generation One (Merchandise)
The Decepto-Pack was sold by HG Toys in 1985. It's the only actual transforming Transformers "costume" marketed to the public. Why they chose the Decepticons over the Autobots is unknown, except perhaps that bad guys are just cooler.
Packaging variant: The Decepto-Pack was sold featuring either a cheerful blond boy in a yellow shirt, or a sullen brown-haired boy in an orange shirt. The boxart differed completely between variants. Presumably the brown-haired version came later, since it matches the live model.
The face on the Decepto-Pack shield kind of resembles Cy-Kill from the Tonka GoBots series.