> What's the benefit of this type of stacked PRs over a well-curated set of commits, and reviewing per commit?

What I can see is that you can easily append commits to e.g. the first PR in a stack, which would insert them into the middle of sequence of commits.

This will require rebasing and fixing the subsequent PRs in a stack the same way you'd need to rebase and fix the subsequent commits in a mega-PR. But it makes the right thing easy (keeping all the commits to the foundation of the change together) rather than makin the wrong thing easy (appending fixup commits across the entire change in a random order so that the actual foundational change is lost).

Keeping all the foundational commits together also keeps all the discussion over the foundational change together.

You could argue that you'd want to only do the foundational PR and stop, but doing the whole stack of PRs gives the reviewers more information about where you're going, and allows work to continue asynchronously.