I've always done this by just creating new PRs that are based on the branch of the old one. Maybe this is a UI improvement?

Yes, you can do that, but there's lots of things about it that are awkward. This makes it a more first-class feature.

Yeah that is a “stacked” pr. The tooling is nice especially when have a larger stack and make changes to the first branch. Update refs + push all branches, same with merging and rebasing onto main.