The main subject of the parsha is the law of annulling vows. This fits with the inner explanation of vows: regarding an upright person, the Sages say, “What the Torah prohibited is enough for you,” because one must refine physical things rather than reject them. Only someone for whom there is concern that physical matters may bring him down needs a vow. Therefore, it is understood that the ultimate purpose is not the vow itself, but, on the contrary, the annulment of the vow.
It is necessary to understand why, at the conclusion of the section on vows, the Torah says, “These are the statutes… between a man and his wife, between a father and his daughter in her youth, in her father’s house,” and does not mention the main law of vows: “He shall not desecrate his word; whatever comes out of his mouth he shall do.”
The explanation is: the basic law that one must fulfill a vow is self-understood. It is also implied from many stories in the Torah that the Jewish people already knew that a vow must be fulfilled. Therefore, we must say that the novelty of our parsha is not the law of “whatever comes out of his mouth he shall do.” That statement is only an introduction to the main law: the annulment of vows by a husband or a father.
Accordingly, we can understand why Moshe spoke this section to the heads of the tribes. This alludes to the law of annulling vows through a beis din. This indicates that the main subject of the parsha is the annulment of vows, not merely the obligation of “whatever comes out of his mouth he shall do.”
This idea — that the main point of the parsha is the law of annulling vows — fits with the inner explanation of vows. It is explained that regarding an upright person, the Sages say, “What the Torah prohibited is enough for you,” because one must refine physical things and not push them away. Only someone whose spiritual state is not as it should be, and where there is concern that physical matters may bring him down, needs a vow. Therefore, it is understood that the goal is not the vow, but rather the opposite: the annulment of the vow.
This is why the law of annulling vows is stated close to the Jewish people’s entry into Eretz Yisroel. While they were in the desert, the Jewish people were separated from the world. But when they entered Eretz Yisroel, the process of refining the world began. Therefore, the section on annulling vows was stated then, so that they would be able to refine the matters of the world.
Nevertheless, the idea of “he shall not desecrate his word” was also stated close to their entry into Eretz Yisroel. The lesson is that one must ensure that “he does not make his words mundane” — that involvement in worldly matters should not be in a mundane way, but rather one should draw holiness into them.