The purpose of the article is to talk about the arbitrary and sometimes cruel nature of the legal system. They have power because they are known to have power, because what lawyer will overstep the boundary implied by the almighty power bestowed onto the judge to effectively ask, “will you please just do a different ruling?”

They don't invoke anything, beyond a simple appeal to humanity. And the fact that they work, when everything else about the legal system is based around mechanical certainty, is terrifying for a lawyer; because it undermines the system they're trained to understand as “if X then Y will (almost) always happen.” If a judge can just decide, “you know what, instead of Y I'll decide to do Σ,” then that shakes the implicit trust of the system. By revealing that the Justice System is based around the whims of a small group of people with disproportionate power.