It's about common practices. A way of 'knowing' what the route is for.
While you can do anything you want (you could even udpate a record using a GET request), it's common practice (And there's probably someone who has more insight than I have) that a PUT request is used for updating the record, while a POST request is used to create a new record.
So while you can use them interchangeably in a functional perspective, it's not a good practice, as other developers may try to use PUT requests to update a record and it would not allow it.
Again, this is just the common practice, and I'm pretty sure someone can explain the actual rules about it more detailed than me. But that will at least get you going.