I was a manager for a short while and the only underperforming employee was transferred to a team of his choosing since he had a long history good reputation overall and just seemed to dislike my particular set of projects.
However, I quit soon after. During the PiP period, I realized that the reason I was on a PiP in the first place was because I really didn't want to be there and had subconsciously started slacking off. I had previously been a high-performing employee, but the company changed. Found a job I preferred and here I am.
Poetic justice was them calling me a few months later to consult on a problem they couldn't solve on their own :-)
Both teams I was on did scrum, with point values that denoted how much time a given task should take. Under both PIP's I had to finish a full X week's amount of work, with the end of each sprint being a hard deadline.
There were regular, sometimes bi-weekly meetings to see that I was on the right track, that were either performed by the managers themselves or someone from HR.
One lead to me quitting 4 months after, and one actually lead me to having a stronger connection to my manager / team. I'd say it's sort of a wake up call saying "Are you really committed to this project/team/company?"
I was on one where I passed. I thought it was quite demeaning how my manager, the GM and HR waved their finger at me out of the blue as if they were angry parents who felt they needed to scorn someone. I didn't truly understand what I was in trouble for ESPECIALLY how much they emphasized "don't try to rope others into this." I didn't say anything in the meeting other than I apologize for this performance and I agree to the PIP ... In the next meeting my manager said my silence was noted by HR as a lack of empathy and they were close to escalating to instant firing at that stage. I wasn't exactly sure what I was expected to do but I attended all necessary meetings and improved my performance and was taken off the PIP. At this stage I'd gone through the recruitment process with other roles and resigned shortly after.
EDIT: As OP is a manager they might want to know how they can manage poor performance better. The worst manager's I've had have skipped weekly/monthly meetings with me then after three months exploded about certain work which was mentioned in a passing conversation not being completed. If you are considering escalating performance issues to HR I think it is good practice to ensure the staff member is notified after the meeting in email that you've talked to them about performance.
It is challenging to coach someone who does not care. There are people who take employment for granted, who don't seem to have a survival instinct or believe another job is just waiting for them. Trying to empathize with those who don't see the world the same way I do is an ongoing challenge; figuring out how to motivate someone requires understanding who they are which is non-trivial giving varying life experiences.
I've reported an employee to HR once for some ongoing inappropriate behavior. While they recovered and our relationship recovered and went on to get a promo, in retrospect I should have talked to him 1:1 and attempted to fix it without HR.
Unemployment benefits are strictly a function of being let go for cause or not for cause. You might be on a PIP for cause-related reasons (absenteeism, showing up drunk but otherwise contributing), or non-cause-related reasons (the maladroit who took over as your manager got told to he needed to downsize his team). The presence of a "PIP" is unimportant. What is important is the documentary burden employers have to meet to disqualify you for unemployment, which in California can be substantial.
Even if they have a really good reason for firing you for cause, you might still get your full unemployment if they deem it less expensive than fighting with you and the state over it.
I worked for a well known defense force contractor and worked under someone who was put on a PIP. The PIP was just a formality in the overall scheme of things as they couldn't fire them without putting them through one first. It was a death march with no chance of success. It was hard hearing them go through it as they were a nice enough person, just not at all suited to the role they were in. In short, PIPs were not seen as methods to actually get an employee back on track in their current role or another one; they were already fired, it's just got a ~12 month notice.
In another organisation a coworker of mine was absolutely useless and were put on a PIP. They genuinely tried to get them back on track and the PIP lasted for a very long time and even renewed their contract when they had a perfect opportunity to let them go. Ultimately they weren't fired directly, when their contract came to its end they put the job up for applications and they didn't win it back. I didn't find out about their PIP until years after the fact and TBH I wish the team knew about it. For all this time we thought they were getting away with their terrible work ethic.
Maybe you have a genuine, acute but deemed manageable problem, and a manager who genuinely wants to help you overcome and move past it. Maybe.
If not, as others have said, it's a prelude to firing you. If you don't seem to be failing out, they will find a way to adjust it until you do. In the meantime, they are collecting data and paperwork to "justify" your eventual termination.
So, it's time to look elsewhere.
Oh, and, any severance will likely be tied to a non-disclosure/non-disparagement agreement. If you don't want to be caught by one of those, leave before they terminate you. Or have enough available assets to be able to not depend upon the severance.
Keep in mind, if they manage to terminate you "for cause", you may not be eligible for unemployment insurance benefits (in the U.S.). This can cut both ways: Because each claim contributes to raising the rates the company pays for the unemployment insurance participation, they may want to deny you said claim as a cost-saving measure. On the other hand, such denials can incur substantial legal and staff-hour costs, when they are fought. So, if the company is reasonably well off or "well-managed / best practices", they may decide it's simpler and more straight-forward -- "cleaner", and also maybe lowering risk of retaliation -- to just terminate you "at will" and let you collect unemployment.
If your health insurance is from the employer (again, U.S.), you should be able to participate in COBRA extension of benefits (at your cost), regardless.
----
P.S. I'm no expert. If you're in this situation, verify these things for yourself.
In this limited experience, 100% of the PIPs were just a grace period before firing them. One of them just found something else in the meanwhile, the other one -an idealistic chap- fought until the very end a war he couldn't win. His case wasn't about performance, but about what he thought was right. A big loss for the company as he was an amazing engineer that just needed some guidance, but he got a mediocre manager with who he clashed...