The Ramban at the end of Bo is a classic work on Jewish philosophy and probably the most quoted Ramban in Chumash. It’s well worth seeing inside. Here’s a summary:
Reason for the Plagues
The Ramban says that from the time of Enosh there were three types of heretics: 1) Those that didn’t believe in G-d at all; 2) Those that believed in a G-d, but didn’t believe He knew what was happening in the world; 3) Those that believed in G-d’s knowledge, but didn’t believe that He oversees the world or that there is reward and punishments.
By favoring the Jews and altering nature through the plagues, the falsity of the heretical views became clear to all. The supernatural wonders indicate the world has a G-d who created it, knows all, oversees all and is all powerful. And when that wonder is publicly declared beforehand through a prophet, the truth of prophecy is made clear as well, namely that G-d will speak to a person and reveal His secrets to His servants, the prophets, and with acknowledgement of this principle the entire Torah is sustained. (The Ramban brings down a number of pesukim supporting this.)
Reason for so many Mitzvos regarding the Exodus
Now, because G-d does not perform a sign or wonder in every generation in sight of every evil person and disbeliever, He commanded that we should have constant reminders and signs of what we saw in Egypt and we should transmit it to our children thoughout the generations. G-d was stringent in this matter as we see from the strict penalties regarding eating Chometz on Pesach and neglecting the Pesach offering. Other mitzvos regarding the Exodus are tefillin, mezuzos, remembering the Exodus in the morning and evening, Succos.
There are also many other commandments that serve as a reminder of the Exodus (Shabbos, the festivals, redemption of the firstborn,…). And all these commandments serve as a testimony for us through the generations regarding the wonders performed in Egypt, that they not be forgotten and there will be no argument for a heretic to deny faith in G-d.
The Reason behind Mitzvos in General
When one does a simple mitzvah like mezuzah and thinks about its importance, he has already acknowledged G-d’s creation of the world, G-d’s knowledge and supervision of the world’s affairs, the truth of prophecy and all the foundations of Torah. In addition he has acknowledged G-d’s kindness towards those that perform His will, for He took us from bondage to freedom in great honor in the merit of our forefathers.
That is why Chazal say, be careful in performing a minor commandment as a major one, for all of them are major and beloved since through them a person is constantly acknowledging his G-d. For the objective of all the commandments is that we should believe in G-d and acknowledge to Him that He created us.
Purpose of Creation
In fact this is the purpose of creation itself, for we have no other explanation of creation. And G-d has no desire, except that man should know and acknowledge the G-d that created him. And the purpose of raising our voices in prayer and the purpose of Shuls and the merit of communal prayer is that people should have a place where they can gather and acknowledge that G-d created them and caused them to be and they can publicize this and declare before Him, “We are your creations”.
This is what the sages meant when they explained “And they shall call out mightily to G-d” as from here you learn that prayer requires a loud voice for boldness can overcome evil.
Everything is a Sign of Hashem
Through recalling the great revealed signs of Hashem of the Exodus, a person acknowledges the hidden signs of everyday life which are the foundation of the entire Torah. For a person has no share in the Torah of Moshe unless he believes that all our affairs and experiences are signs from Hashem, that there is no independent force of nature regarding either the community or the individual.
Reward and Punishment
Rather if one observes the commandments his reward will bring him success and if he transgresses them his punishment will destroy him. Hidden signs of Hashem can be more clearly recognized as regards the affairs of a community as in the predictions in the Torah in the matter of the blessings and the curses as it says – And the nations will say, “For what reason did Hashem do so to this land…?” And they will say, “Because they forsook the covenant of Hashem, the G-d of their forefathers”. This matter will become known to the nations, that this is from G-d as their (the Jews) punishment. And it is stated regarding the fulfillment of the commandments, “Then all the people of the earth will see that the Name of Hashem is proclaimed over you, and they will revere you.”