|Art by Mel|
The Mishkan is called the Mishkan HaEidus, the residence of testimony. The Medrash says that the Mishkan is testimony to the fact that Hashem created Heaven and Earth.
The Sfas Emes [Rebbe Yehudah Leib Alter] explains that in this world Hashem is hidden. We cannot see Him or His greatness in every part of creation. We don't know why things were created or their connection to Hashem.
The Mishkan is the link between Shamayim and Aretz [Heaven and Earth]. Each part of the building of the Mishkan and its Keilim [vessels] represent things in this world and reflect the face of Hashem hidden behind them.
Therefore seeing the Mishkan and understanding the purpose of it, is the greatest testimony to the fact that Hashem created a very spiritual world hidden behind seemingly mundane objects of nature.