Q. I have often wondered if we make hatarath nedarim on erev Rosh Hashana, why do we recite Kol Nidre? Do any of the two declarations actually work?

A. The source for the Kol Nidei declaration is the Mishna in Nedarim (23a-b) “If one desires that his nedarim should not take effect, should declare on Rosh Hashana that all his nedarim to be expressed until next Rosh Hashana, should not be valid.” Rosh (Yuma Ch. 8: 28) explains this to be the reason for reciting Kol Nidrei in Yom Kipur, when everyone attends shul, as we also find (Yechezkel 40: 1) that Yom Kipur is also named Rosh Hashana. He adds that this declaration will be effective only if at the time of saying the neder, one does not recall saying that declaration. Rosh (ibid.) after quoting Rabenu Tam that Kol Nidrei is a declaration for the future, mentions that the earlier tradition was to consider Kol Nidrei an annulment on nedarim past. There is also the opinion of the Nimukei Yosef who maintains that Kol Nidrei is not an annulment or future declaration but rather a prayer to Hashem for not being punished for unkept promises. Zohar (Ra’ayah Mehemna 3: p. 255) asserts that the purpose of Kol Nidrei is to repeal and annul all dinim and negative decrees against Am Yisroel.
Sh’loh Hakadosh explains that we perform hatoras nedorim on Erev Rosh Hashana for the purpose of being z’rizim makdimim or diligent in preempting mitzvos. Besides, it is necessary since people may not be careful to recite, intend or understand properly what the intention of Kol Nidrei is. Sheilas Yavetz (1: 145) indeed wonders why the need for the two declarations and offers other elucidations.
Remoh (Y. D. 211: 1) rules that Kol Nidrei serves as a declaration for future nedarim and you can rely on it in case of great need.
Minchas Shlomo (Nedarim 24a) debates if for the purpose of nedarim that were created by observing a mitzva or a minhag, the above declarations will help, since when those acts were done, there was no intention of them becoming a neder. See Igrois Moishe (Y.D. 1: 127) as to what kind of minhag or adopted practice may become a neder.
Horav Shlomo Miller’s Shlit’a opinion is to be lenient on adopted traditions that are not a mitzva and rely on the above declarations without needing further hatoras nedarim. (See questions 67, 360, and 688 in this forum)

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a.