I’ve only worked in the public sector and have only had to PIP people a few times. It’s almost always a sign that you need to get out. I’ve only done it when I’m at wits end with a person. They all ended up quitting before we got to the 90 days but yeah as said it’s basically a CYA move. Likely the people are low performing enough by some measure you can fire them but it’s sort of a last chance thing. And it documents where they need to improve and what they need to do, so all that is good for the employee if they want to improve (and I do believe nobody wants to do a bad job).