Why does the Torah insert here the phrase "Ki Savo't el Eretz Moshvoseichem"?
Rashi: To reassure them that they (K'lal Yisrael) would enter Eretz Yisrael.
And why did Hashem insert the Mitzvah of Minchas Nesachim here?
Ramban: Having promised the children that they would enter Eretz Yisrael, He reassured them that nothing would occur to prevent this from happening by teaching them Mitzvos connected with Eretz Yisrael.
Seforno: Until the sin of the Eigel, they were able to bring Korbanos without a Minchah and Nesachim; 1 From the Eigel and onwards, it became necessary to bring a Korban Nesachim together with the Olas Tamid 2 - which was a Korban Tzibur - for the Korban to attain the level of Re'ach Nicho'ach. And from the sin of the Meraglim and onwards, even a Korban Yachid required a Minchas Nesachim to validate it.