Tehillim-The Pesach Psalm-78 II-Alshich
The 13th of Nisan is the Yahrtzeit of Rav Moshe Alshich: “I will expound a theme, hold forth on the lessons of the past, things we have heard and known, that our fathers have told us. We will not withhold them from their children, telling the coming generation the praises of God and His might, and the wonders He performed (Psalms 78:2–4).”
You might say that there is no point in telling you what is already written in the Torah, and that therefore it is better to tell of things not written in the Torah but which have occurred at a later time.
However, Asaph, I feel that if I were able to tell you just of events that have occurred in the world says as examples of behavior to be avoided in order not to be punished, I can still not be denied the right to describe actual events that have been described in the Torah.
To the argument that many others could quote from events described in the Torah, and that besides, no one would deny any of that what has been documented in the Torah, so what need is there for you, Asaph? He responds by saying that all the others have told those stories to aggrandize God, not to distill their moral ethical content as is his intention. There is therefore ample justification for Asaph to cover known territory. (Alshich HaKadosh, Romemot El)
Haggadah: We repeat this story year after year not only to aggrandize God but to distill their moral ethical content.