Fent is cheap and potent, and so some dealers add it to their heroin (and a couple other drugs, fake pills are also common). Then you can add more cutting agents (like flour), and stretch it even further. However, because it’s so potent (50x compared to heroin) a tiny “hot spot” of badly mixed heroin could kill even an addict. As I understand it, deliberating making people OD is largely a myth. While it might get some addict’s attention, it also gets the attention of the police.
It’s so potent that cross contamination is a major issue. Drug dealer cuts their heroin on a dirty table, next kind of drug gets cut on the same dirty table. For at least some cases, no one meant for fent to be in there.
But it doesn’t produce as good of a high as other opioids do (I don’t understand the mechanics I’m afraid). As I understand it, the demand is mostly heroin addicts who are really deep in.
Interestingly, from what I remember, it’s actually somewhat similar to heroin in that way. Morphine is supposed to be a better high (again, for people not super deep in), but isn’t as available. So people use the cheaper and more potent heroin.