User blog comment:Sorairo/Comparisons and World Building….and Questions/@comment-9090085-20141009081003/@comment-67.1.120.162-20141009180113

it may be too Blue Gender-y but I always imagined the Grimm emerging as a natural sort of population control for Humans; if humans overtook the world before the Grimm came they could have done a lot of damage to the ecosystem, prompting the rise of the Grimm. They find their prey through negativity--selfishness, exploitation, opression, along with more basic feelings like anger or sadness. In come the Grimm, destroying the ancient world, and in a dark-sort of way, resetting the world back to it's natural state.

Of course, when it was the Grimm that became too numerous, Humans discovered Dust; so now light and dark hold one another in a precarious tension. I think the show makers hinted that if something were to happen to the world's supply of dust, it would be very, very bad for the human race on Remnant.