Ask HN: How does California 90-day Probationary starting period work?

In short, I'm in my probationary period, which I don't think I've ever been before -- and I'm old as rocks, so....that seems strange to me. Nonetheless, I'm curious what, if any, implications there are for either side. I'm pretty used to California law being 'at will' -- meaning, 'everybody do whatever you want whenever you want for any reason or no reason at all good, good luck, stay chill'. in my case i'm looking to bounce, but...why would a company even have a probationary period?