In my homebrew, they are the keepers of the Aunn - the lifeforce of nature. The Aunn is a cycle of birth, life, death and rebirth that continually feeds the natural cycle. Things like civilization (at the city level) and undead disturb this cycle, preserving things that were meant to be created and destroyed in a never-ending cycle. to druids, without the cycle things would fall apart - if one does not return from death, then eventually nothing will remain, and without death things cannot be reconsituted into new life to change and grow - and eventually one will grow tired (or bored) of a life that has no end or hope of change. The latter is the state of the gods, and they are considered to stand outside the Aunn, but they are disliked by druids for the souls they pull out of the cycle to dwell in afterlives of bliss or torment.