User blog comment:Zathronas/Analysis by an author: Volume 2 episode 9: Search and Destroy/@comment-10390252-20141003134556/@comment-67.1.251.16-20141003215048

My current theory of the Grimm is that they are the yin to humanity's (and faunus kind's) yang. Kinda like Galactus in the Marvel universe, the Grimm represent a constant of reality: in their case, darkness must exist alongside the light. So the Grimm have a purpose in the world of remnant, serving as the main driving force behind humanity's rapid progress and  development (I'm speaking of the way Remnant is now, not the ancient past when the Grimm first assaulted mankind). The Grimm will therefore never really go away or be hunted to extinction (there might even be severely negative consequenses should that ever happen--throw the whole of creation out of balance or something like that)

I like to think that the Grimm came into existence along side Humans to serve as their antithesis. While humans are inteligent and resourseful, but are also capable of savagry and stupidity, the Grimm are mindless and chaotic, but also able to evolve and learn and gain inteligence. Humans have souls (representing the sense of self and individuality) while the Grimm are "manifestations of anonymity"--they have no individuality or sense of self. Humans have mortal bodies that, when dead, return to the physical realm (circle of life!) But, according to Pyrrha, (if I'm interpreting her lil' prayer right) humans also have immortal souls that live on after death. The Grimm (apparently) have immortal or at least very-long-lived bodies (provided nothing kills them through violence) but no soul or anything that continues to exist after they die; their bodies evaporate, returning to the void.

Oobleck stated that the Grimm can be drawn to negativity; I concede that I don't really have a way to incorporate that into my theory, other than an anology of moths being drawn to light; the Grimm are drawn to darkness (though in a more peotic sense)