Wondering why husbands can void a wife's promise to YHWH? (Numbers 30) Also wondering WHY a wife would tell a husband about a promise she is making to YHWH??
Respectfully, the problem I think you are having is that you are understanding Numbers 30 with your English language mindset of our day (naturally!) These verses, regarding the husband's authority to nullify a vow made by his wife seems disturbing to us.
But, to understand Numbers 30 we really need to invoke what a Hebrew of that time understood about the supreme importance of vows and oaths and how the family unit was designed by YHWH to function. The way YHWH established the family and expected it to operate is not at all like we manage families in our time. Similarly, we only have our experience and understanding of the meaning of a vow or oath, which is quite different than that of the Biblical times.
According to YHWH, when a man makes a vow, he is obligated to keep it. Period. It is that important. Numbers 30:2. This is also true of a woman who is not a wife, Numbers 30:9. But when the family unit is involved, here is where the family hierarchy established by YHWH enters the picture. The hierarchy is this: A child is to submit to their parents, the wife is to submit to her husband, the husband is the authority over the family - and he submits to YHWH.
The husband, of course, is supposed to be honorable and in good-standing, and a man of YHWH, so this YHWH-given authority is not meant to be permission to abuse his wife or children. Put in a way that is a little less disconcerting to our modern western society, a wife is under the covering and authority of her husband, and the husband is under the covering and authority of YHWH - thus so is the wife. Nevertheless, we find in scripture that the man is the head of the household and has the authority to nullify any vow made under his roof by his wife or children.
Realize that Numbers 30 also permits the husband to accept any vow made by his wife, so we need to look at what these verses mean to imply since the intent of the vow is not addressed in Numbers 30. It's not that the vow might be an evil vow or an irresponsible vow that cannot possibly be kept. It's that in Biblical times, (just as we do today), we sometimes plead with YHWH and make a vow to Him, often unthinking and rash: "Oh, Lord, I'll not do that again", or "I promise to never cuss again", or "I'll never ask you for anything ever again".
Vows like these obligate the person (and may obligate the household), and in the case of the wife, the husband can, and should nullify them if necessary. But a vow made by the wife which is righteous, and honorable, and can be kept, the husband can let it stand (Numbers 30:7, 11, 13-14). This would be the way a good, Torah obedient household would have operated.
As to your concern "wondering WHY a wife would tell a husband about a promise she is making to YHWH?", I'm not sure how to respond. A righteous household, I think, would not encounter situations where the wife felt she should make a vow or an oath without her husband's knowledge. She has, after all, submitted to her husband's authority. (Remember to think ancient Hebrew mindset, not modern western mindset). This family structure is important to YHWH, but is lost on society today.
And finally, let me add that there is a distinct difference in the Bible between an oath and a vow. An oath imposes an obligation upon the one making the oath. A vow is by definition a conditional promise.
Oaths usually are made in the Name of YHWH. The Hebrew term is: "nishba 'be-YHWH", and is an oath made in YHWH's Name, for example an oath between two people. But an oath made TO YHWH is: "nishba 'le-YHWH", which is literally "swear TO YHWH".
Both forms are obligatory but they can both be made between people, or directly to YHWH. Today, we don't take oaths and vows as seriously as YHWH intended them to be. That is rather sad. In days of old, one's word was his bond. It's not so in our time. In or western society, no one is taught the importance or difference between an oath and a vow, and we just "do what is right in our own eyes."