Because Om is the sound-form of Brahman, those who know Brahman always begin their acts of sacrifice, charity, and austerity by uttering Om. This is not superstition. It is alignment — connecting the act to its ultimate source.
Think of a royal seal stamped on a document. The seal does not change the content, but it authorizes it. In the same way, uttering Om before a sacred act connects that act to Brahman and gives it a dimension beyond the merely personal.
Every puja, every havan, every sacred undertaking in a Hindu household begins with Om. This practice traces directly back to the teaching Krishna gives here.