Bageshwar Dham Sarkar
Why is truthfulness so important?
What truth gives the one who speaks it
One who speaks truth fully gains vaak siddhi (the power that one's spoken word comes true) without any mantra or guru. If he declares that the Paramatma must appear somewhere, Brahm has to appear.
Without chanting any mantra and without going to any guru, he gets vaak siddhi. This is the benefit.