Ramban, in his introduction to Bamidbar, explains that the laws of the Israelite encampment in the desert parallel those given to the people before the revelation at Mount Sinai: Just as at Mount Sinai, no one was permitted to approach the mountain when God’s presence was there, so too the encampment surrounding the Mishkan, which contained God’s presence, required the same restrictions. Ramban’s comments here are based on the principle which he establishes in the beginning of Parashat Terumah: The Divine Presence which rested on Mount Sinai transferred to the Mishkan, and the very structure of the Mishkan is based on the revelation at Sinai.
In fact, according to Ramban, the entire Chumash can be seen as a description of the encounter between God and the Jewish people. In the beginning of Shemot, Ramban explains that the construction of the Mishkan is included in Sefer Shemot because it represents the completion of redemption: When the Mishkan was completed and the Shechinah rested among the Jewish people, the return to the status of the Avot was achieved. Bereishit, Shemot, Vayikra and Bamidbar thus all revolve around achieving and maintaining God’s presence in our midst. According to Ramban, the divine revelation of Sinai was not a one-time event but an ongoing one which was sustained through the Mishkan and then the Mikdash. Rambam too writes that the sanctity of the Mikdash, which was based on the presence of the Shechinah, is everlasting (Hilchot Beit HaBechirah 6:16). This is why, according to Ramban, there is a lasting mitzvah to remember the revelation at Sinai (Sefer HaMitzvot, shichechat halavin 2).
Based on this, we can understand why we read Parashat Bamidbar the week before Shavuot. The fundamental idea of Sefer Bamidbar is that the revelation at Sinai is not merely a past event. Before Shavuot we recall that God’s presence remains with us even through our travels and travails in the desert.