Rashi (citing R. Yishmael): In order to break between the current Parshah and the following one, because otherwise, it would seem as if Moshe told them the Parshah of Nedarim that follows, but not the Parshah of Mo'ados.

Ramban #1: Because it is common for the Torah to add a Pasuk to break between two Parshiyos in this way. ¹

Ramban #2: In order to point out that, although the current Parshah includes Korbanos, Moshe is addressing the whole of Yisrael ² , and not specifically the Kohanim ³ - bearing in mind that they were about to enter Eretz Yisrael, and all of Yisrael will be commanded to offer the Temidin, the Musafim and to bring their Nedarim and Nedavos ⁴ Moreover, a major function of the Parshah, is to warn them to refrain from Melachah on the Mo'adim, which applies to all the people equally.

Ramban #3 ⁵ : To draw a distinction between the Mo'adim and Hataras Nedarim, inasmuch as, as opposed to Hataras Nedarim, which requires only one Yachid Mumcheh or even three Hedyotos, Kidush ha'Chodesh requires a Beis-Din comprising three Mumchin. ⁶

Ramban #4: Because we might otherwise have thought that "Zeh ha'Davar ... el Roshei ha'Matos" in the next Pasuk, refers, not to the Parshah of Nedarim that follows it, but to the Parshah of Temidim and Musafim that precedes it. ⁷