GitHub documentation states:
After a pull request is opened, anyone with read access can review and comment on the changes it proposes.
In public GitHub repositories, what is the point of allowing people other than the repository owner or collaborators (that is, people who don't have write access) reviewing pull requests?
I suppose hypothetically it could be a learning opportunity, but it seems far more likely to be a waste of time. The people with write access get to decide what goes in and what changes need to be made in the code for that to happen. It's highly unlikely that a random read-access-only developer will know exactly what the owner wants.
Also, what would be the proper etiquette in this situation for the pull requester and the owner?
Sometimes a non-collaborator is a former contributor or other subject matter expert who has knowledge about the code or algorithm and can be invaluable as a reviewer. In an open source project, this could be an emeritus member of the core team, or for a company, it could be a colleague who has moved to another team and no longer has write permission on the repo. I've actually requested such reviews from former core team members before.
I agree that in many cases it's possible for a rando to pop up to an open source project and give a useless review, and I see those myself. I usually just ignore them, or if a particular party is providing lots of unhelpful reviews, I usually deal with it like I deal with any other unhelpful behavior and ask them nicely to stop.