"All Parshiyot are created equal, but some are more equal than others." (A paraphrase of George Orwell’s Animal Farm.)
Parshat Bo is one of the most important parshiyot in the Torah. Firstly, Rashi thinks that it would have been appropriate for the Torah to begin from this week's Parshah. (Bereshit 1:1) Secondly, Parshat Bo forms the basis of the philosophical underpinnings of the entire halakhik system. But wait - there's more! Thirdly, the Parshah also tells us about the Kabbalistic concept that the Jewish people are partners in creation with HaShem. All of these three ideas actually come from the same mitzvah - Kiddush HaChodesh. (Maybe it is because of Divine Providence that this idea came to me on Rosh Chodesh Shvat.)
Rashi states that one would have thought that since the Torah is a book of laws and Mitzvot, that it should have started from the first Mitzvah that is given to the Jewish people - the Mitzvah of Kiddush HaChodesh in Parshat Bo. (Anyone who wants to ask about the different Mitzvot that were given in Sefer Bereshit - the short answer is that those Mitzvot were not given to the Jewish people. Rather those Mitzvot were given to individuals. In other words, those individuals were not necessarily included in the brit of Torah and Mitzvot.) If we step into the Torah time machine, we can go back in time about 2000 years and observe how the Jews determined what day was Rosh Chodesh in the time of the Sanhedrin.
Rosh Chodesh is the new moon that happens every month. When two kosher witnesses saw a sliver of the new moon in the sky, they would go to Beit Din and tell them that that day was Rosh Chodesh. (For more information, see the first two perakim of massechet Rosh HaShanah.) So very nice, now you see why Rashi thoguht that the mitzvah of Kiddush HaChodesh was so important - but why is it so important to halakhah and Kabbalah?
WARNING - potentially mind altering idea, read ahead at your own risk.
I am forced to reveal a secret about halakhah, the rabbis were given the authority to determine what is a mitzvah d'oraita. Did anyone faint? Am I in cherem? This mitzvah proves the collaborative relationship between HaShem and the Jewish people - HaShem wrote the Torah and invested the Jewish people with the mission/ability to transform the Torah from legal theory to reality. In other words, HaShem gave us the authority to interpret and enact the Mitzvot. (Frummie disclaimer {aka I am not Blu Greenberg} - there are limits to this Rabbinic interpretation, but the Jewish people are collaborators with HaShem on the system of Torah and Mitzvot.)
Kiddush HaChodesh is one of the best examples of this collaboration between HaShem and the Jewish people. HaShem said the Pesach is to be on the 15th of Nisan. But who determines whether the 15th of Nisan in a particular year should be on a Monday or a Tuesday? Beit Din are the ones that determine which day would be Rosh Chodesh Nisan, and thus they also would determine which day is Pesach. Suppose that there was only one pair of witnesses that saw the new moon on a Monday. If Beit Din determines that they were not kosher, then Pesach is on Tuesday and not on Monday. Suppose that HaShem caused the new moon to shine on Monday so that Pesach would be on Monday for that year. The human Beit Din overrules HaShem, and Pesach and all of its Mitzvot d'oraita are not on Monday, but rather they are on Tuesday.
To sum up - HaShem wrote in the Torah that Pesach will be on the 15th of Nisan, but HaShem needed (and needs) the Jewish people to be collaborative partners to make the Torah a reality. This is part of the philosophy of halakhah - the halakhot in the Torah are/were created by HaShem to be enacted in this world via human beings. That is to say that halakhah is part human interpretation and part Divine revelation.
(A quick note on my using the terms “human beings” vs. “the Jewish people.” It would seem from the first couple perakim of Bereshit that Adam, Chava and Noach were not specifically Jewish. Indeed, had any of them fully succeeded in their missions - perhaps there would not have been any need for the Jewish people to have a special brit with HaShem. Ultimately, HaShem chose Avraham and his descendants to be the nation that was chosen to accomplish HaShem’s mission in the world. Basically, HaShem has one mission for humanity, to perfect the world. Originally this mission was given to all human beings, and later it was given exclusively to the Jewish people.)
Kabbalah explains that there is a different kind of collaboration between HaShem and the Jewish people. Halakhah focuses the legal aspect of the collaboration - it focuses on the Torah as a law book. Kabbalah focuses on the mystical aspect of the collaboration - it focuses on the Torah as a mystical guide book. HaShem has a mystical blueprint for the world, and it is the job of the Jewish people to make that blueprint a reality. The Jewish people are partners with HaShem in the ongoing mystical creation of the world(s).
Now we can see why Parshat Bo is an important Parsha. Kiddush HaChodesh is the first mitzvah in the Torah and according to Rashi it would have been an appropriate beginning point for the Torah. Not coincidentally, the first mitzvah in the Torah also establishes the legal foundation of the relationship between HaShem and the Jewish people. HaShem has given the Mitzvot to us, but it is our job alone to enact them properly in this world. Finally, this same relationship exists on the mystical level. The Torah that HaShem gave us is a mystical blueprint for the world (interpreted on the Remez and Sod level of the Torah) and it is our job to enact the mystical reality in the world.