Achdus Like at Kabalas Hatorah:

Rav Moshe of Kozhnitz zy"a (Sefer Daas Moshe) writes similarly that Parshas Netzavim is always read before Rosh Hashanah to teach us that we must stand united before Hashem on the Yom Hadin, just like we stood as one at Har Sinai.

In this vein, he explains the subsequent verse (29:12): “In order to establish you today to him as a nation and He will be to you your G-d." "Today" refers to Rosh Hashanah. If we are a singular, united nation on Rosh Hashanah, then He will be our Master who will push away all harsh judgments and protect us.