I don't think that's what he means. Some deaths would make it impossible to resurrect a companion - if the body wasn't intact (be it explosion or desintegration) or some death spells the characters would be dead - dead. I don't know if it's still in 5e. I am not terribly fond of those (failed save against this spell RELOAD) but I imagine it would be beneficial if we couldn't resurect our companions out of some situations - like vampire spawn sucking our companion dry.