Sometimes the primary UML diagrams, such as class and sequence diagrams, are not a perfect match for capturing certain details about your system. Composite structures help fill some of those gaps. Composite structures show how objects create a big picture. They model how objects work together inside a class, or how objects achieve a goal.
Composite structures are fairly advanced, but they're good to have in your bag of tricks because they are perfectly suited for specific modeling situations, such as showing:
Show the parts contained by a class and the relationships between the parts; this allows you to show context-sensitive relationships, or relationships that hold in the context of a containing class
Show how a class is used on your system with ports
Show design patterns in your software and, more generally, objects cooperating to achieve a goal Composite structures provide a view of your system's parts and form part of the logical view of your system's model,