Each sefer of the Chumash is unique, and the Ramban explains the unique quality of Sefer Bamidbar.
The Torah elucidated the laws of the offerings in Vayikra, it begins now the lay down the mitzvos that were commanded regarding the Ohel Moed, the Tent of Meeting where the offerings were brought.
The Torah already warned about the impurity of the Sanctuary and the sacrifices, but now it sets aside the Mishkan in the desert like Har Sinai was set aside, being that the Divine Presence rested there. In regards to the Mishkan we were warned “A stranger who approaches will dies.’ Similarly on Har Sinai the Torah said “Do not approach for you will be cast down.”
Regarding the Mishkan we were warned about gazing at the holy area, and at Har Sinai we were warned about seeing the holy radiance of the Shechina. The Kohanim guard the Mishkan and they also approached Har Sinai in sanctity.
Hashem commanded the Kohanim how to guard the Mishkan and its vessels, how to camp around it and keep a distance. This is all out of respect and honor for the holiness of the Mishkan. As the Ramban explains in parshas Terumah, the Mishkan was meant to continue the Har Sinai experience as the Jews traveled.
After enlightening us in the relationship of the Mishkan to Har Sinai the Ramban concludes: Sefer Bamidbar is full of mitzvos that were commanded specifically for that time the Jews were in the desert, as well as relating the miracles which happened to them in order that the wondrous acts of Hashem should be made known. The sefer describes conquering Klal Yisrael’s enemies and how to divide Eretz Yisrael. This sefer has few mitzvos for the future generations except for some sacrificial laws which were not fully explained in the previous seforim.