Earlier quoted context omitted.
The structure is to keep police officers honest which is more important than a particular person going to jail. The operating principle is that a dishonest police force is more dangerous than a criminal who was caught using illegal evidence. Typically the criminal will commit another crime and if you get evidence against them legally, then you can prosecute them for the new crime.
The "we both go to jail" policy would probably do even betting at keeping police officers honest. The threat of the officer going to jail being significantly worse than the threat of allowing the criminal to walk free. The details are sketchy, who prosecutes the police/prosecutors? But in principle I think it makes sense.
Not if the officer going to jail is being manipulated by their superiors or is otherwise arranged to take the fall for someone else.