I think this is maybe what you’re asking about: with PRs, you often respond to feedback by adding new commits on to the PR. This is because code review is based on reviewing an entire branch. In a stacked diff based system, you respond to feedback by amending the commit. This is because reviews are tied to individual commits, not branches.

The reason people prefer this is that it keeps changes small and focused, and makes sure you have a high quality history. It also lets you assign different reviewers to different parts of your stack, which is helpful for all sorts of reasons. It also means that you can land earlier commits while waiting on review for the later commits, instead of holding it all up at once. Lots of stuff like this.