From The Heretic Knowledge Vault
Portal mages seem to have a certain uniformity of appearance -- male, bald or shaved head, flowing robes, posture of meditation/concentration when the gate is about to operate. They may be in deep magical trance when they assume this posture. Alternatively, they may simply be putting on a show to impress the yokels.
We don't know exactly what they do or how they do it. However, according to Sarine, a "much smoother transference" results from having them around, rather than having the teleportation device function automatically, as the elven travel platforms do. ( Jon, who hates warp gates, isn't impressed.)