Right, a PR is "just" a set of commits (all must be in the same branch) that are intended to land atomically.

Stacked PRs are not breaking up a set of commits into divisible units. Like you said, you can already do that yourself. They let you continue to work off of a PR as your new base. This lets you continue to iterate asynchronously to a review of the earlier PRs, and build on top of them.

You often, very often, need to stage your work into reviewer-consumable units. Those units are the stack.