The symbolism behind this is the search for truth a difficult, long, and often painful process in and of itself, as any seeker can tell you. And, much like a piercing, the truth marks you inside, with enlightenment and sorrow, instead of outside with bars and metal rings. Twin vertical labrets, through my lower lip. Small steel fangs, in a manner of speaking the Serpent's Wisdom is a key part of the Satanic mythos. Truth in hearing, truth in sight, truth in speech and taste, truth in touch, and even truth in scent.
Truth is worth a little pain.