For me the benefit is that I can revert or cherry-pick things one entire PR at a time, and I don't have to care if the author implemented their PR with a bunch of small "work in progress" commits.

And GitHub at least sets the author of the squashed commit as the one who opened the PR, not the one who merged it.

I can definitely see where it wouldn't work well for other workflows but I've had it work well on several teams and it seems easier than trying to get everyone to clean up their commits into nice, clean, well-titled histories before putting up a PR.