The system has a task to determine whether a person has actually committed a crime, and how serious. You don't want to happen to be a false positive. Nobody does.

But with plea bargain, I bet the number of false positives is somehow elevated.

Also, prisons are a terrible way to handle criminals. Instead of reforming the criminal and helping reintegration into the law-abiding society, many prison systems, including the US's, produce hardened criminals who learned a lot more of criminal trade during incarceration, or crushed people who have no chance to get to their feet again. Unless a society just kills every criminal, it makes sense to care what happens to them, because they'll be back.