You can view a full example here:
Introspection can be used for different purpose, the one presented in 'Dive Into Python' is simply a way to add functionality (plug-in) dynamically in your application.
By dynamically I mean without doing modification in the core application to add a new feature.
Taking the 'Dive Into Python' example - a simple app to extract attribute from file of different file - you can add the handling of a new file format without doing modification to the original application.
I recommend that you finish the book. As you read, everything will become increasingly clear.