If your PRs are more than one commit, you've already introduced work that builds on top of previous work before it's been reviewed.

Stacked diffs just let you review one commit at a time, rather than requiring a full review of the entire thing, while still letting you see that these changes are related, so you can go look at the full context if you'd like.

This lets you do things like "hey, backend person, please review this backend commit, frontend person, please review this frontend commit" instead of "both of you review this full PR". This can save the frontend person time if, for example, the frontend code passes review, but the backend reviewer hasn't had time to review the backend changes yet. They won't be pinged by the other review.