They're generally used to prevent a lawsuit, but they do not prevent lawsuits. A layoff on the other hand...

Because no matter what you write in a PIP someone could turn around and sue you for it and have a good enough case to at least force a settlement. Everyone has some sort of "protected class" they are part of.

But with a layoff it's really hard to prove the layoff was targeted unless everyone in the layoff happens to be part of the same protected class.