Outline notes can be used when naturally arising clusters occur in Linked notes. When I realize that I have a conglomerate of notes around a common topic, I create a structured outline creating a cohesive story.

On the other hand, outline notes can be used for intentional clusters, e.g. while reading a book. A good book will cover several ideas, structured in a specific order to form a narrative. Each idea should be contained in its own note, since Notes should be atomic. All the notes related to the book can be collected in an outline note, ordered in the way the author of the book structured the content, to serve as a summary.

One of the beautiful things of using atomic linked notes, is that a single note can be part of multiple hierarchies. This gives flexibility to some of the Problems of hierarchical taxonomies.