The first Rashi in Sefer Bereishit is a very famous one. Rashi asks why the Torah starts with the creation of the world as opposed to the first Mitzva of the torah, “Hachodesh Hazeh Lachem?”
The Siftei Chachamim comment that Rashi is not questioning why all the material between the creation of the world and the Pasuk of “Hachodesh Hazeh Lachem” is included in the Torah, but why the Torah starts with the creation. In other words, Rashi is suggesting that the creation of the world should be mentioned at another point in the Torah.
Rashi answers his question by stating that by beginning with the creation of the world, the Torah transmits the message of the Pasuk from Tehillim (111:6) כח מעשיו הגיד לעמו לתת להם נחלת גוים. Hashem tells us of His ability to give us the estate of nations. In other words, if other nations come and say that we are wrong for conquering the seven nations who lived in Eretz Yisrael, we can say that the world belongs to Hashem and He chose to give the land to us.
You can also look at Rashi another way. The reason why the Torah starts with the history and the development of the nation of Bnai Yisrael is so that we would not have a guilty conscience, that we unjustly took the land from the seven nations.