The special case of the widow, respecting vows, serves to show the personal responsibility of each character. It is the language of the scripture that everyone shall receive his own reward according to his own labor. 1 Corinthians 3:8.
Related Commentaries of Numbers 30:9
Numbers 30:9
9 But every vow of a widow, and of her that is divorced, wherewith they have bound their souls, shall stand against her.