Why does the Torah insert this Pasuk here?
Rashi: To inform us that, despite the fact that Yisrael strayed from before Hashem and made the Eigel, Hashem instructed Moshe to lead the people to Eretz Kena'an.
Ramban: Refer to 10:10:2:2*.
Why did Hashem mention inheriting the land without any reference to fighting for it?
Had they not sent the Meraglim, they would have received Eretz Yisrael without as much as raising a sword. Refer to Devarim, 1:8:2:1.
Why did Hashem see fit to mention the Shevu'ah here?
Seforno: Because, if not for the Shevu'ah, due to the constant rebellion of Yisrael, they would not have merited to inherit Eretz Yisrael.