I am so attached to this central verse because it manifests in one line a connection to the entire course of Jewish history. David Moss's Haggadah perhaps illustrates this better than any other Haggadah text I know of, using mirrors placed in the center of various scenes of Jewish communities throughout history, literally placing the individual amongst the freedom narratives of the nation throughout time. While it is not my personal stance, I would go as far to say that one need not believe that the actual Exodus occurred in order to fulfill the mitzvah of seeing oneself כאילו הוא יצא ממצרים, and this would be an honest reading of the line. As someone who has always been attached to the rational elements of life and particularly to history, this passage tangibly connects me with my ancestors and gives me a halachic imperative to connect with their stories. As Isaac Abravanel states in his commentary to the haggadah:
For it is impossible that a person living in our age of exile will not experience [in his life] some type of suffering... For not only was it our forefathers that He redeemed in that all-encompassing redemption, but us as well He redeems and saves every day from different troubles, just as He did for them. And therefore Scripture says, “and us He took out from there” (Deut. 6:23). It did not say “them” but “us” — because each one of us is redeemed innumerable times in his lifetime in the course of having to live in exile.
The line בכל דור ודור in the haggadah, indeed the entire maggid section of the seder itself, originates from a central commandment in the Exodus narrative — והגדת לבנך ביום ההוא לאמור, זה עשה לי בצאתי ממצרים (Exodus 13:8).
But of course I wasn't there. You weren't either. How do I fulfill this mitzvah?
The seder itself explores this very thesis when it comes to explaining the zeroa on the seder plate. We purposefully do not raise it, because this was not the pesah sacrifice that our ancestors conducted. Rather it is a symbol of such (see Bavli Pesahim 116b, specifically commentary of Rashbam).
But we come to a drastically different conclusion with reciting and examining the Passover story with our children — we say that story, and we act it out as if we were there. But there's another step which we add to the symbol of the "capital E" Exodus. In one word, keilu, we transfer the experience from one event which is by definition impossible to remember personally, to a continuing narrative of escaping each of the straights in the overflowing narratives of the Jewish people.
This word reinforces even more the human agency that is necessary in telling the story of Passover. What is the correct way to do it? There necessarily is not one way. More emphatically, it is specifically not enough to say words of the Haggadah, but one must expand on the text in her own personal way. Questions are at the center of this experience, not the four that are scripted, but the ones which the parent provokes from the child, that we bring out of each other (Bavli Pesachim, 116a). Only by doing this, will one fulfill the mitzvah of doing so keilu she left Egypt. In a tradition that has a series of codified liturgies and texts, בכל דור ודור חייב אדם לראות את עצמו כאילו הוא יצא ממצרים mandates that the words of the Haggadah are important, but are not enough on their own. In the words of the Rambam in his comments about the mitzvah of conducting a seder (Sefer HaMitzvot, 157th Mitzvah, commentary on Exodus 13:8):
...וכל מי שיוסיף במאמר ויאריך הדברים בהגדלת מה שעשה לנו השם ומה שעשו עמנו המצרים מעול וחמס ואיך לקח השם נקמתינו מהם ובהודות לו ית' על מה שגמלנו מחסדיו יהיה יותר טוב. כמו שאמרו כל המאריך לספר ביציאת מצרים הרי זה משובח.
Expounding about the Exodus, both the original story and subsequent ones is praiseworthy.
The story of the Exodus has served as a central theme for liberation movements across generations and ethnicities. For twentieth and twenty-first century Jews, בכל דור ודור literally serves as a bumper sticker slogan across our t-shirts (because after all, what's a Jewish cause without a t-shirt?) In every generation, each year, we must remember that we were slaves, and collectively act to bring ourselves out of both our personal and collective Egypts. The story is ongoing, and there is an obligation to apply it to the contemporary situation each and every year. As Michael Walzer states:
"The story is more important than the events, and the story has grown more and more important as it has been repeated and reflected upon, cited in arguments, elaborated in folklore. Perhaps that was the intention of the authors: certainly, they urge the repetition often enough. The Exodus belongs to a genre of religious and legal texts designed for public reading and rereading and for analogical application." (Exodus and Revolution, (New York: Basic Books, 1985), 8-9)
The Exodus narrative has resonance both for the Jewish people and for anyone that claims the Bible as a Holy text. Indeed, monotheistic religions across society see themselves as if they personally left Egypt, and utter a pedagogic challenge to their children to engage in such introspection, as well.