What is this matzah for?

Does the rabbi have a “pashta” for this central passage in the Haggadah?
This matzah that we eat, for what reason? For what reason, in the days of our fathers, it was not able to leaven until the King of kings, the Holy One, blessed be He, and their Redeemer, appeared to them.
What moment of revelation is this? If the plague of the firstborn is meant, then they did not leave until morning. If the departure in the morning is described here, then they were driven out by the Egyptians, as the verse itself says, and not by God.