i see this tradition and the tying of the string as a reminder to one self that each of us have a personal responsibility to our parents, our sisters and brothers, our children to live a meaningful life, not to live a reckless life, to take care of our self, to care for the people that need help, to set example to others.
I believe this tradition started shortly after our forefather Khon Boulom of Tai/Lao preached to his seven daughter in laws before they relocate into the new land of Savanaphoum. Most of his teaching can be found in the chanting of Baisi Soukuan.