science

You basically guessed it. Fentanyl is crazy cheap to produce, way more potent than heroin, and because of that you don't need very much to cut other drugs with it. It gets mixed into everything because it's a cheap way to make the drugs feel better and has the added bonus of being ultra addictive. It sounds crazy that they're even cutting uppers like meth and cocaine with it but if done right the user doesn't even realize it's there, just enough to add some euphoria and make you really want more the next day. It's fucking poison.

Quick edit to add: in pure form it's very dangerous. Just a big poof of the dust from a bag breathed in could be fatal. Very hard to accurately dose for users at such high potency.