With at-will employment is a pip necessary? What happens if you fire someone who is performing?

The issue is unemployment payments. In many states, if you fire someone without cause then you have to make the unemployment payments. If you fire with cause then you don't.

I've seen a few workers take the employer to court and the company usually settles as it doesn't want the reputational damage and court costs so they just pay up. People just assume the worst of the employers when these court cases become public if even it is later dismissed.

People can take you to court for anything; pip is good way to protect against claims of wrongful termination. I agree in principle that at-will should make this unnecessary, but the reality of a litigious society makes pips a good move.

Specifically regarding at-will employment and the United States, you can still be sued for discrimination based on race, gender, and all other protected classes. PIPs and other similar processes are primarily created, from a legal/HR perspective, to protect against those lawsuits.

So, somewhat counterintuitively, at-will employment in the United States(especially in states like California) is not actually at-will in a practical sense do to the threat of discrimination lawsuits.

Yes. Some at will states support lawsuits