What observations argue that IV acetaminophen should be a component of multimodal pain management, whereas IV opioids should be "rescue" or "pain breakthrough"-alleviating agents?

What clinical observations argue for the position that IV acetaminophen should be a foundation component of multimodal pain management, whereas IV opioids —which while effective, are also associated with a range of problematic side effects — should be considered so-called “rescue” or “pain breakthrough”-alleviating agents?