We use this feature extensively at $dayjob.

Imagine you have some task you are working on, and you wish to share your progress with people in bite sized chunks that they can review one at a time, but you also don’t want to wait for their reviews before you continue working on your task.

Using a stacked set of PRs you can continue producing new work, which depends on the work you’ve already completed, without waiting for the work you’ve already completed to be merged, and without putting all your work into one large PR.