Hagada preparation

Tuesday is Rosh Chodesh Nissan. Thus we begin to prepare the study of the Hagada.
Why do we include a study of 4 types of people at the beginning of the story ?

We have a mitzva to tell the story to all in a way that will make a most powerful impression. Thus we need to prepare with thoughts of who will be at my seder so that I know what approach to focus on in our words.
Now may be a good time for a gem of Torah study.