Throughout literature the vow has been important. Nietzsche made a lot of hay from the idea of a man who was so noble and strong as to make promises and keep them. You think what's important here is keeping the promises. But it's the other part, making promises, that is a sign of the ubermensch. Salman Rushdie has made plots turn on the vow, and he's only a recent example in large novels of the force of the vow, which frequently is magical.
Of course in monasteries and religious orders everywhere the vow is central, even if not effective in making the oath-taker a better person.
For the community, a vow has a special character, for it binds people together for the sake of an idea that transcends them all.
Presently we take no vow. It seems worth including, however, in the annual re-commitment ritual. It deepens one's living into the idea, and strengthens the convivial bonds between people, which are prone to dissolution under the pressure of sad human emotions: anger, envy, jealousy. The vow carries the easily distracted over the rougher patches of the river.