Why does the Torah insert the words "Va'asu ba'Aretz"?
R. Bechayei #2: Because they were about to enter the land.
RS"R Hirsh: Yisrael inherit the land only in order to bring the Torah to total fulfillment!
What did Hashem mean when He said to Moshe "ve'Atah Poh Amod Imadi"?
Rashi: Following His instructions that the rest of the people return to their 'tents' (their wives).
QUESTIONS ON RASHI
Rashi writes that "Amod Imadi" obligated Moshe to separate from his wife, Shabbos 87a says that Moshe decided to separate from his wife due to a Kal v'Chomer (from Bnei Yisrael, who needed to separate)!
Riva (Shemos 19:15): Once Hashem commanded Yisrael to separate, Moshe divorced his wife. Aharon and Miryam said that had he not done so, Hashem would not have commanded him, for Moshe had prophecy a long time before this, and Hashem did not make him separate! 1
The Gemara says in Pesachim, 18b that even something that one can learn from a Kal va'Chomer the Torah will insert. Consequently, the fact that the Torqah writes it is no contradiction to the fact that Moshe decided to separate from a Kal va'Chomer.
Riva: Even though Tziporah said 'from when Hashem spoke to him, he separated from me', that was a temporary separation. He divorced her only after Bnei Yisrael needed to separate.