Why does the Torah insert the (otherwise superfluous) word "Vaya'as es ha'Kerashim la'Mishkan"?
Rosh: If Yisrael will sin, the Mishkan will be a Mashkon (a security) for them 1 - (It will be destroyed instead of them).
Rosh: When Moshe asked Hashem what will be when there is no Mishkan or Mikdash and Yisrael sin, Hashem replied that He will take the Tzadikim as a security.