Why do Buddhists take vows?
Question for Geshe-la
Geshe Pema Tsering; Geshe-la's answer:
"Not only Buddhists take vows; followers of other religious traditions also take vows. They vow to restrain from misconduct. That's what 'taking vows' is, to restrain from bad actions, harmful activities, to restrain from non-virtue.
In Buddhism there are many different classes of vows; there are fully-ordained monks' and nuns' vows, novice vows, lay practitioners' vows.
Vows are taken not only to restrain from harmful actions, but also to accomplish their opposites - beneficial actions. And the reason we do so is because the result of harmful actions - non-virtues - is suffering, and the result of beneficial actions - virtuous actions - is happiness.
And, as Buddhists, we take vows from our Lamas, conceived as one with the Buddha, to quicken our realization of Enlightenment, so that we can best benefit sentient beings."
Source
http://www.buddhahouse.org/Teachings-Archive/why-do-buddhists-take-vows.html