It's basically trying to bring the stacked diff workflow pioneered by Phabricator to GitHub.

The idea is that it allows you to better handle working on top of stuff that's not merged yet, and makes it easier for reviewers to review pieces of a larger stack of work independently.

It's really useful in larger corporate environments.

I've used stacked PRs when doing things like upgrading react-native in a monorepo. It required a massive amount of changes, and would be really hard to review as a single pull request. It has to be landed all at once, it's all or nothing. But being able to review it as smaller independent PRs is helpful.

Stacking PRs is also useful even when you don't need to merge the entire stack at once.