1)
Why did Moshe need to declare "Zeh ha'Davar asher Tzivah Hashem"?
1.
Rashi: He wanted to cast off any suspicion that what he was currently doing was for his own Kavod or for the Kavod of his father's family, but because Hakadosh-Baruch-Hu had commanded him to do it.
2.
Oznayim la'Torah: To stress the importance of the impending declaration of Yisrael 1 declaring Kadosh Aharon and his sons, the Mishkan and all its Keilim. 2
3.
Yoma, 5: The key words here are "Zeh ha'Davar". To teach us that even reading the Parshah of the Milu'im was crucial to the Mitzvah. 3