As a geochemist, I am often interested in one specific mineral (or several minerals) contained within a rock, because chemical and isotopic differences in that mineral will reveal important clues about where the rock came from, how it was generated, and when.
However, to go from a rock the size of my fist to an individual mineral grain requires breaking the rock down. The first step along that path for a fist-sized rock is the jaw crusher. As gravity pulls the rock piece further down into a funnel-shaped opening, a jaw mechanism will repeatedly open and close, and will crush the rock into little bits.
For coarse-grained rocks, the jaw crusher may be sufficient to get single-grain sized pieces, but for medium- and fine-grained rocks, a second crushing step is almost always necessary.