Purpose of probationary period?

I'm curious what the purpose of a probationary period is, specifically within an at-will employment state. If an employer (or employee) can legally terminate employment, at will, at any time, why even bother with a probationary period? Consider your probationary period indefinite.
I'm about 11 weeks into a new job and the employer has a 3-month probationary period. Any thoughts? Employees get full benefits from day 1 (except vacation and sick time, which starts at the 6-month mark); however, we also get a week of PTO immediately.
Seems... Unnecessary to me. Maybe more documentation is required to let someone go, even in an at-will employment state, when not under a probationary period. Any thoughts?
I'm about 11 weeks into a new job and the employer has a 3-month probationary period. Any thoughts? Employees get full benefits from day 1 (except vacation and sick time, which starts at the 6-month mark); however, we also get a week of PTO immediately.
Seems... Unnecessary to me. Maybe more documentation is required to let someone go, even in an at-will employment state, when not under a probationary period. Any thoughts?
Comments
Good guess but they still need to pay into uneployment for any employee on the books.
The goal for everyone is to find a place where they can fit in, and that's the company's goal too... they don't want to invest a lot of time training someone in their processes and procedures (and potentially expensive technical training) for someone who ultimately isn't going to pan out.
From your perspective It's also to make sure the company is a good fit for you. It's in place for both parties.
I wouldn't be worried about it.