But it turns out that prison time makes criminals more likely to reoffend further down the line, because prison is basically school for sociopaths. The longer you spend inside, the less you learn about how to run a Dairy Queen and the more you learn about severing toes and sending them in ransom notes. Australia found that mandatory minimums were associated with increased crime rates, because if you treat people like career criminals, that's what they become.
Mandatory minimums also ruin a judge's ability to exercise discretion. Take the case of Tonya Drake, who was offered $100 to mail a package for a stranger. She agreed because she needed the money to feed her kids (and had apparently never seen any crime show ever). The package contained crack, of course, and Tonya had technically trafficked drugs. She was sentenced to a mandatory ten years for making a dumb decision in a moment of desperation. The judge said, "This woman doesn't belong in prison for ten years for what I understand she did. That's just crazy, but there's nothing I can do about it."
So that's where we're at. We have laws so ridiculous that they make judges feel powerless.