Why do we have a mishkan?
When the Jews left Egypt, they were in a limbo status. On the one hand, they were descendents of Avraham, Yitzchak, and Yaakov, who had previously made covenants with God. However, on the other hand, these covenants only involved God and an individual; these were not covenants between God and nation. Thus, the Jews who left Egypt worshipped God as individuals not as a nation. Consequently, each individual had the status of eino metzuvah v’oseh. God sought a nation to worship him; He needed to create one.
In order to create the Nation of God, everyone underwent a mass conversion at ma’amad Har Sinai. (Ramban, introduction to Parshas Terumah). Jewish conversions have several elements: (1) a bris milah; (2) a tevilah; (3) acceptance of the Torah; (4) korban. Nowadays, without a bais ha-mikdash, we do not require korbanos for conversion. This process was modeled on the mass conversion at Har Sinai. (Ramban ibid. Yevamos 47b; Kerisos 9a). The main purpose of any conversion, and especially the original conversion, is the acceptance of the Torah.
Each individual now incurred an obligation to observe the Torah, and, thereby, all of the individuals constituted a single entity: am Yisrael. Thus, the conversion created the nation of Israel; the acceptance of the Torah affected the conversion.
God designated this nation as His own nation, His am segulah. Having established their nationhood, am Yisrael deserved God’s constant presence. He therefore instructed Moshe in the building of the mishkan, so that God could rest His Presence on the Aron. (Ramban, ibid.). God’s Presence rests on the Aron because the Aron houses the Torah, and the Torah is the source of Jewish nationhood.