The purpose of the system is what it does. Here, drug laws result in tens of thousands of wrongful arrests, some portion of which will take the plea deal even though they are innocent. Typically because they are low income and need to get out of holding so they don't lose their job.

Instead they get a record (for nothing) and risk prison. They cannot afford to prove their innocence.

Don't forget the 'trail tax'. Sentences are 10-20-50 years if you exercise your right to trial, while if you 'voluntarily' waive your right the plea put before you could be 1-5 years. Even if you are innocent, you are now at the point while in jail you already lost your job, your car, your house, are you going to risk that a system that did that to you already is going to give you a fair trial? Or are you going to go off of past experience that system is going to fuck you and just take the shorter bit?

John Oliver did a fantastic segment on that and called it torture AFAIR.