Packages are used to group UML elements such as classes and use cases. You may want to review those chapters for more detail about showing the contents of a package. Class diagrams are covered in Chapter 4; use case diagrams are covered in Chapter 2.
One of the most important applications of package diagrams is to view dependencies in your system. Other important high-level system diagrams include component diagrams, which show the key software pieces, and deployment diagrams, which show how the pieces get deployed to hardware. Component diagrams are described in Chapter 12; deployment diagrams are covered in Chapter 15.