There doesn't need to be anything sinister about it. Fentanyl is cheap and potent. You can take cheap, low quality heroin, mix it with a little fentanyl and sell it for more profit as high quality heroin.
However, if it's not mixed perfectly, you end up with "hotspots" of fentanyl. If someone ends up with a hotspot they can easily overdose. The occasion overdose also attracts more users because it gives the impression of potent, high quality heroin.