NOTES for Num 29:40-30:16
The rule in the Book of Numbers concerning vows made to God seems like a simple tribute to the relationships established in patriarchal society, where authority in a large patriarchal clan, or even in a small family, belongs to the head of the clan, the father, the husband. In reality, however, everything is not so simple. The point is not that a woman is less responsible before God than a man, and therefore, if she is not alone, all her vows must be confirmed by one of the men: her father or her husband. The point lies in the relationships that bind a person to God and people to one another.
In the Bible, the attitude toward vows and oaths, especially oaths in God's name, is generally ambiguous at the very least. On the one hand, without oaths, by which all contracts and transactions were sealed in antiquity, no formal civil and legal relationships would have been possible at all. Therefore an oath and a vow were accepted as a necessary and inevitable part of public and religious life. On the other hand, those who made vows were sometimes seen as placing themselves under a kind of spell, one with a fully magical character and meaning, so that the guarantor of the promises made was no longer God but a certain force contained in the very words of the vow.
At that point this was no longer Yahwism but paganism, and in its worst form, where there was simply no place for anything other than magic. It is no accident that Jesus forbids such oaths and vows in the Sermon on the Mount: for a Christian, a simple "yes" or "no" should be enough. Everything else is from the evil one; it is an attempt to rely on something beyond human control, making what is beyond human control the guarantor of the firmness of a promise made by a person.
Meanwhile, fallen humanity in every age has wanted guarantees, and reliable ones, preferably absolute ones. But not everyone and not always has understood that guarantees must be paid for, above all by surrendering freedom in relationships with the one from whom one wants those guarantees. By binding himself with a vow, a person wants guarantees from himself; by binding another with an oath, he wants them from his neighbor. And in any case God is expected to act as surety, regardless of whether He approves such oaths and vows or not.
The situation is strange, at the very least. This is why, over time, a person's right to oaths and vows is limited, disappearing altogether with the coming of the Messiah. The limitation on the vows of daughters while their father was alive, and of wives while their husband was alive, was only the beginning of the process, a perfectly natural beginning: to bind oneself by a vow without asking the one who is responsible before God for you or for the family would mean giving obligations primacy over relationships, which is something to be avoided with all one's strength if normal, full spiritual life is desired.