The purpose of problem analysis is to define the resources that support the problem domain.
The scope of problem analysis includes the purpose and the interfaces of the classes that represent these resources. So far, you have the classes and their purpose.
But what method do you use to justify the interfaces for these resources?
Interaction diagrams provide a tool for translating use cases into a sequence of communication between specific objects.
This communication then translates directly into the object interfaces.
The result is an explicit justification for the interfaces you define for each problem domain class.