As your understanding of the problem domain increases, your conceptual model becomes more complicated and can reach an unmanageable size. You can use packages to divide your model into smaller, more manageable subsets.
A package is the basic organizing element of a UML system model. You can think of the entire system as a package that contains all the other packages, diagrams, and elements. One package can contain subordinate packages, diagrams, or single elements, and you can set the visibility of a package as well as the visibility of the elements it contains.
You can use packages throughout a development process to group related elements. For example, if a conceptual diagram becomes too large, you can partition its elements using packages. Or, you can use packages to define system architecture.