Give a large collection more room
Convert a Chest when it has grown into a substantial collection of its own and you want it to have more space in your overall organization.
Keep related bookmarks together
Conversion is useful when the Chest already represents one clear topic, project, or area of interest. Its bookmarks stay together while the collection becomes easier to find as a Cave.
Change your structure as your needs change
You do not have to decide the perfect structure at the start. Begin with Chests for smaller groups, then promote one to a Cave when it deserves a top-level place.