The answer Rashi brings to the question of the midrash carries an important message, that the land of Israel belongs to the Jewish people by right. However, it is difficult to see how this idea answers the original question of what the connection is between the accounts recorded in the book of Bereishis and the fundamental purpose of the Torah, which is to serve as a book of divine instruction for the Jewish people, teaching them the mitzvos that they must perform. I would like to present two explanations of how this answer does, in fact, explain that connection, and then demonstrate how these two explanations merge into one.
The Ramban, in his commentary to parshas Acharei Mos, writes that the mitzvos are meant to be performed mainly in the land of Israel. This applies, according to the Ramban, not only to agricultural mitzvos such as terumah and ma'aser which relate specifically to the soil of Eretz Yisroel, but even to mitzvos such as tefillin and tzitzis, which are mitzvos that relate primarily to the person. Rabbi Eliyahu Meir Bloch explained that even though, in turns of our obligation to perform these mitzvos, there is no difference between the land of Israel and the lands outside of it, in terms of the effect that the mitzvos have on those who perform them there is a difference. Since God's presence is more intense and evident in Eretz Yisroel, the effect that the mitzvos performed there have upon us is also greater. Thus, since all the mitzvos are enhanced when performed in Eretz Yisroel, it seem appropriate that before the mitzvos are commanded to the Jewish people, its claim to the land, which is the natural place for those mitzvos to be performed, be clarified.
Another explanation can be found in the Keli Yakar, by Rabbi Shlomo Ephraim of Luntshitz, the famed rabbi of Prague in the early seventeenth century. He writes that our claim to Eretz Yisroel needs to be clarified because otherwise the nations of the world would challenge the Torah itself.How, they could claim, can Moshe tell the Jews in Egypt to avoid theft and take a lamb of their own to serve as the Passover sacrifice, when He did not care that the Israelites were going to rob other nations of their land? In order to show that God's laws are based on justice, the claim of the people to the land needed to be clarified and firmly established. For this reason, the Torah begins with the book of Bereishis to demonstrate that the land if Israel does, indeed, belong to the Israelite nation.
I believe that the two answers we have presented, one based on the Ramban, and one taken from the Keli Yakar, are complementary to each other. Rabbi Yosef Dov Soloveitchik, of blessed memory, has pointed out that any unauthorized use of God's world on our part constitutes theft. God created the world, and it belongs to Him. Man is allowed to make use of that world only on the condition that he follow God's rules in doing so. If man sins, then his use of the world constitutes theft. This is the meaning, according to Rabbi Soloveitchik, of the words we say during the final prayer service of Yom Kippur, the Neilah. In the Shemoneh Esreh of Neilah we declare that God has taught us how to confess our sins so that we refrain from theft. Why should theft singled out here, more than any other sin we may have committed? Rabbi Soloveitchik explained that theft here represents all sin, because all sin is, in reality, theft, because it is a misappropriation of God's universe. The Torah, then, is the Jewish people's guide to the proper use of God's universe, instructing it how to avoid any misappropriation of it. Within this context, it is important to demonstrate that the Jewish people has not misappropriated the land of Israel, the optimum location for the performance of the mitzvos. As we embark on a new year, following the intensive period of the holidays in the month of Tishrei, in which we endeavored to cleanse ourselves of any various acts of misappropriation we may have committed over the course of the past year, it is important to reacquaint ourselves with this fundamental idea, that our only right to benefit from the universe is if we do so in accordance with the guidelines set forth for us in the Torah. A proper study of the book of Bereishis will provide us with this orientation, and it is, perhaps with this purpose in mind that Rashi began his commentary to the Torah in the manner that he did.