Some companies do unlimited vacation like this. It's a perk.
Other companies use this to be restrictive on the vacation people take. It keeps more people working more days.
This speaks a lot to company culture.
I was cynical, but year later I'm pleasantly surprised to be proven wrong - I think we collectively all took 15% more holiday. Not a mind-boggling change, but not the "we'll all be bullied to stay at our desks" result I'd anticipated.
Anecdotally I think the largest change is people get less precious over guarding their holiday as a precious resource, even if they've no idea how they'll spend it. You finish your week's work on Friday morning? Ask if anybody needs anything from you and then just take the rest of the day off. No compulsion to find some busy-work and protect 4 hours of vacation.
My current job offers a few "extra" things on top of unlimited PTO like during the summer, a $100 bonus if you post a pic of some activity you did on PTO (literally anything, and can even opt out of sharing if you are so inclined) or more recently, a "competition" for the best use of PTO. I think someone reading a book in a hammock won.