Composite Objects
They come in handy if a composition of single objects is conceptually of the same class again. You can then build hierarchies of collections which are very easy to deal with.
Anti-example: a group of persons is not again a person (but a group) and has different properties.
But if you glue several of our bodies (cubes, cylinders, etc) together, you will end up with a new body (albeit a more complex one). The new entity is a body in its own right, and has all the required properties (volume, surface, color, etc).
The “Composite objects” design gives you a way of dealing with the collection of objects in the same way as with a single object.
If you read the Objectivity manuals, you will see a long treatment of Composite Objects there.