The Literal Commandment
"Let them construct a sanctuary for Me, that I may dwell among them. According to all that I am going to show you, as the pattern of the tabernacle and the pattern of all its furniture, just so you shall construct it. (Exodus 25:8-9)
The literal commandment given to the Israelites was for them to construct a "sanctuary" (Hebrew miqdash: a holy place). Since G-d is a Holy G-d He does not just show up and "hang out" with the Israelites. His Holiness requires a holy space in the midst of the Israelite camp.