PIPs, at least from what I’ve seen in 12 years at DoE, are a way for managers to retaliate against employees without raising the ire of the (completely useless) union.
I've never heard of a PIP actually succeeding (an employee improving to the satisfaction of their manager) so it delays the inevitable, perhaps allowing the employee to quit and find another job before their fired. It's definitely meant to avoid a wrongful dismissal lawsuit but I think that almost never works; either the employee is going to sue or they're not, "being given a chance to improve" isn't going to change that.
If there are explicit goals or metrics to be measured, which I have seen in many cases, it's pretty clear what needs to be done to clear it. If they don't meet those goals, and they're attainable (e.g. other people in the same role are performing that well on average), it's a pretty clear defense for a lawsuit.
The whole place was a clown show that I was unhappy at anyway, so while I entertained the idea of taking them to court, when I realized the PIP is basically an elaborate attempt to defend against wrongful termination cases I ultimately just found a much better job.