RIFTS posits a handful of well-known archetypes for human augmentation, and one of them is the chemical route. Juicers are chemically-augmented soldiers, usually used as Special Forces assets and commonly deployed as snipers and assassins, whose chemical enhancements bring them super-human capabilities in physical might and endurance at the cost of burning out the body within 5-7 years of completing the conversion process. As these are the facts behind this fictional system, I can only conclude that this process came about because the state or other entity that needed it had a massive surplus of men willing and able to serve as soldiers, but for whatever reason deemed utterly expendable and unwanted by the oligarchy ruling that state or entity.
In other words, Juicers came about because someone need a lot of jacked-up cannon fodder and didn't want to deal with the consequences of having that surge of veterans re-enter civilian life after the war ended. Long-time gamers may recall another game with a very similar vibe to it--Underground, published by Mayfair--that also had this question come up; the difference, as is commonplace with Palladium products, is the lack of effort spent in answering that question. Oligarchies that are prone to scooping up large segments of their population, using a large subset as a resource for achieving an objective and then discarding it like soiled toilet paper are exactly the sorts that would embrace Juicer augmentation technology.
In the canon, this is said to come out of Europe and I can indeed believe that one or more European states (or factions thereof) would indeed invent and deploy this sort of technology. They would use it to target the young men of their underclasses, recruit them into the military, convert them into Juicers and then manipulate the wars to last long enough for the process to kill them all off within months of the war's end (for all but a handful; all are dead within a year or so). Once the Detox process became viable, this could easily be extended to civilian convicts: accept service as a Juicer in return for your record being wiped clean upon Detox. (Once Cyborg technology gets to a point where swapping brains into shells becomes reliable, "promoting" Juicers to Cyborg shells becomes another viable alternative.)
As chilling as seeing European governments "solve" their "undesirable" problem through exploiting the men--and some of the women--as Juicers may be, I think that it would be far more commonly done in the Americas and Asia once the technology got out there and the means to convert people became easier to establish and maintain within a country; waves of wars would come as one age cohort reached prime draft age, got sucked into it as Juicers and either died in combat or died due to the process burning them out, with only a small core remaining to train the next cohort. The poor of the world, already a prime target for exploitation of this sort, become little more than crops reaped in regular intervals- and the very expendable assets used abroad would become used at home to quell dissension against such exploitation.
The real game-changing point, however, becomes when (by comparison) rather ordinary criminal syndicates become able to field Juicers. Right now we have well-known cartel organizations such as the Medellin of Colombia and Los Zetas of Northern Mexico, groups that were or are sufficiently flush with capital to acquire and maintain military gear and the skills necessary to make the most of it. In this hypothetical situation, once Juicer technology becomes that reliable and replicable these same organizations will be able to take scores of gang-banging street thugs and transform them into superhuman hitmen- utterly expendable superhuman hitman at that, and they will not have a hard time recruiting. The same goes for partisan or terrorist organizations, as they walk in the same circles, and (for the same reason) it's likely to come when this or that intelligence agency flubs an operation and loses a Juicer conversion set-up to a group of that sort.
All this, by the way, is pre-Rifts. After the Coming of the Rifts, there's no reset button- it's all the way back to the worst scenario as soon as someone starts up a conversion operation again because the scope and scale of institutional organization is so small. There's the Coalition & Free Quebec, then Lazlo/Tolkeen/Dwemeor and then Everyone Else- so far as North American goes. Elsewhere is no better, and often a lot worse.
So, what are we left with? The Juicer is a loser. If he volunteered, then he's either a well-meaning sucker or a brain-addled fool. Most likely is that he's forced into it somehow, which means that he's not at all privileged- and likely comes from an underclass or outcast status of some sort. The bravado, the bullshitting, the glamor- it's all ego-armor covering the fact that they're going to die, die horribly and die soon. To be a Juicer is to have No Future; you've chosen to burn out than fade away- so act accordingly.