Moshe prefaced his charge to build the Mishkan with the Shabbat prohibition of doing work to teach Israel this practical point. This is the meaning of the juxtaposition of Shabbat and the Mishkan in the practical realm. In the first description of the Mishkan, which we defined as a theoretical context of God's commandment to Moshe, the juxtaposition with Shabbat acquires additional significance. The aspect of Shabbat that is emphasized in God's command to Moshe is its timelessness. The Mishkan was to be destroyed and replaced by the Temple, which also would be twice destroyed.
Objects are transient and can never serve the function of being an everlasting sign.
What Moshe is being told, on the theoretical level, is that although the people need a sign of God's presence in their midst, neither the physical sign of the tablets nor the Mishkan to house them are that everlasting sign. Rather, Israel's observance of the Shabbat is the sign and the covenant for the ages. Only this non-material covenant can exist as long as Israel and time exist. The Torah does not suffice with the theoretical understanding of both the Mishkan and Shabbat, but it gives practical expression to the theory. The Mishkan was not only described to Moshe in a way that emphasized the Mishkan in theory, it was described in a second pragmatic way in order to be built.