> They fill a similar role to prosecutors that field drug tests serve to cops[1]: they aren't meant to be accurate, they exist to give the the cops legitimacy as they proceed to do whatever it is they wanted to do to you. In the cops' case, they need legitimacy to search and arrest you, and in the prosecutors' case, they need legitimacy to charge and convict you.
In case any of you are interested, or completely flabbergasted as I was, by the idea that law enforcement's purpose and raison d'etre could ever become as distorted and contorted as this comment mentions, you should definitely read more on Walter "Johnny D." Macmillan [1] or watch the movie based on his story: Just Mercy[2].
Absolutely mind-blowing stuff.