A man, when he makes a neder to Hashem or he swears a shvuah to bind a bond upon himself, he shall not profane his word. (Numbers, 30:3)
The laws of Neder and Shvuah are valuable gifts that Hashem bestows upon men.
Neder includes promises of good deeds (such as offerings to Hashem) and also self-imposed prohibitions of certain pleasures or benefits.
Shvuah includes the oath to refrain from some act or to perform a certain act.
When men wish to obligate themselves to give offerings or charity, or to prohibit themselves from acts or from pleasures which could prove harmful, the Neder or Shvuah is very effective as an aid to one's own will-power. Many wrong habits can be corrected, and many righteous deeds can be encouraged, by the proper use of the Neder or Shvuah. Men force themselves by these two stratagems to climb the ladder of Perfection step by step; “Nedarim are a fence for [Talmudic word for a way to prevent breaking one's resolution or a commandment, ed.] abstinence” (Avos 3:13).
This is the first time that the term Isar (which later became Isur) is found to denote the concept of forbidden. Thus “Neder” here means the vow to forbid oneself that which is permissible.
The second form of Neder is that by which one obligates himself to do that which he is not required to do, such as volunteering to bring sacred offering or to perform good deeds which were optional. This second form of Neder is found elsewhere: When you make a Neder... do not delay to pay it... That which comes from your lips you shall keep and you shall do (Dvarim 23:22, 24).
Nothing is mentioned here of the procedure of anulling (literally, loosening in the Talmud, ed.) the Neder or Shvuah by means of a qualified Sage (30:1) (as our nation previously possessed) or by three persons.
Here we have an instance of the Oral Torah which Moshe received from Hashem, and these instances where such important information remained unwritten were intended by Hashem for the purpose of emphasizing the necessity of recourse to the Sages who teach the Oral Torah .
The Oral Torah comes to teach that He shall not profane his word, but others (who are qualified) could nullify his word (Hagigah 10A). It is indeed an essential part of the law of vows, for in some instances the vow may result in serious hardships. Yet this extremely important adjunct is omitted from the Written Torah, and this was so arranged in order to emphasize that Hashem's word must be learned from the Sages and not merely from the Written Torah.
Thus also another major purpose was fulfilled: the imitators of the Torah among the nations should never really understand the Torah laws properly, because the Torah Shebaal Peh was not included in the Scriptures. (Journey Into Greatness)