And Moses did as the LORD commanded him; and the congregation was assembled at the door of the tent of meeting. (Vayikra 8:4)
The Netziv asks: Why was it necessary for all of Klal Yisrael to be present for this particular event? While Klal Yisrael was required to be present at other events, that was to bring about a particular result that was relevant to everyone in Klal Yisrael. This event seemingly was only relevant for the kohanim, as Moshe washed and clothed Aharon and his son. So why did Hashem demand that everyone be present?
The Netziv answers this with an idea which is relevant to our basic beliefs in Torah m’Sinai: We believe that Torah m’Sinai includes the Torah sheb’aal peh (Oral Law). Here in particular the words of the Torah were not fulfilled according to their simple meanings, but with the mesorah of Torah sheb’aal peh. Moshe Rabbeinu gathered all of klal Yisrael together so that they should watch him execute the written words. But he specifically wanted them to see him execute the words according to the mesorah, and not according to what the people would have imagined the words to mean. This was to be a lesson to Klal Yisrael that even Moshe Rabbeinu—who knew what he was doing—could not perform these mitzvohs properly if he only followed the simple reading of the words of the Torah. So Klal Yisrael was called together so that they could witness Torah sheb’aal peh being executed by Moshe Rabbeinu, the one who wrote Torah sheb’kesav (the written Torah).
On the night of the Seder, every family has their on customs and traditions—their minhagim—that they lovingly follow throughout the evening. While it sometimes seems that a minhag does not follow exactly as the Shulchan Aruch reads, we have a rule that we do not deviate from our minhag without ample proof that it is a mistake or a misunderstanding. As we prepare for Seder night, fortifying ourselves in our emunah, it is also our job to uphold our sacred minhagim which are in a sense our Torah sheb’aal peh, passed down from generation to generation. As we fulfill the commandment “v’hagadata lvincha” – telling over the Passover story to our children—we should keep in mind to pass on to them that our traditions, though not from Sinai, they are incorporated within the Torah.