What can stacked PRs do that a series of well-organized commits in a single branch can't?

Stacked PRs tend to encourage a series of well-organized commits, because you review each commit separately, rather than together.

What they do that the single branch cannot is things like "have a disjoint set of reviewers where some people only review some commits", and that property is exactly why it encourages more well-organized commits, because you are reviewing them individually, rather than as a massive whole.

They also encourage amending existing commits rather than throwing fixup commits onto the end of a branch, which makes the original commit better rather than splitting it into multiple that aren't semantically useful on their own.