Israel was to keep the Night to Be Much Observed in part as a night of watchful vigil to commemorate the reason they could leave Egypt so easily: God watched over them as His plan unfolded.
Reading Genesis 15 with the story in Exodus, we can see how God watched over them. Israel's bondage in Egypt had disciplined Israel, preparing them to go through the wilderness, and afterwards, take the Promised Land. This was God's plan for them, and He watched it brought to completion. His greater plan is not completed even now, because we are a part of it! Genesis 17 shows that it has eternal consequences and is still in operation.
The Night to be Much Observed is a significant event in God's plan. Will anyone deny that God watched out for Israel, seeing the blood on the doorposts and lintels and passing over them? Can anyone deny that He watched over them as they finished spoiling the Egyptians during the daylight portion of Nisan 14, watching as they gathered to meet in Rameses?
“Watch" does not mean that God passively observed them as they left. Instead, it means that He actively "guarded" them. "Watched" comes from the Hebrew shamar, used often and translated as “keep.” Whenever one desires to keep something, he guards and protects it. In like manner, God watched, kept, guarded, and protected Israel. Exodus 11:7 shows just how closely God watched, not allowing even a single dog to bark.
Can anyone deny that God watched as the Israelites walked out that night of Nisan 15 in the very sight of the Egyptians who were burying their dead? Most likely, the Egyptians would want to blame the Israelites for the death of their children and animals. They would be enraged. They could not see God, nor blame Him directly, as it were; but they would take it out on His people. But they stood by numbly instead of resisting or fighting.
The Night to Be Much Observed is the official marking of God's watchful care. It is good and right that we celebrate what God did and continues to do. We can easily see that this portion of the first day of Unleavened Bread is of great significance, not just on the basis of its prior history in the life of Abraham, but also its significance to the Exodus. An entire nation of slaves just got up, and without lifting a hand to achieve their liberty, they walked away.
Most people, in order to win their liberty, must undergo bloody warfare, and many people lose their lives. Those who do not suffer the loss of life usually lose their material wealth. Israel did not lose any lives and came away rich! The captor nation was helpless to do anything to retain its slaves because God restrained the Egyptians.
John W. Ritenbaugh
The Night to be Much Observed