How does this workflow work when there are 3 commits that need to be applied in order (A, B, and C), when C gets approved from a code-review standpoint before A and B?
The advantage is that each commit can be reviewed separately. You cannot achieve this with pulled requests as far as I know. I often see people creating a pull request and writing "this depends on PR #1233", the second PR is unreviewable because it contains changes from. The first PR. So the first PR has to merged first... It's a mess. But maybe I am just missing a big part of the PR workflow that solves this problem.
This works better with a system that requires the PR owner to actually hit merge (so it's not accidentally merged into the first PR's branch) and where approvals stick across PR changes (which is undesirable in general, though).
I find it is still useful to include helpful hints like "this PR includes all of PR #1234, if you prefer to review a piece at a time", but that's for the reviewers' sake, not the tools'.