Manannan is at his most powerful in this guise. With his vast knowledge and wisdom, even the Tuatha de Dannan turned to him for help when they needed sanctuary. Manannan not only separated them from the world of men, but provided them with the tools neccesary to obtain immortality. It begins to become clear how ancient Manannan's origins are when viewing him in this guise. Posessing knowledge that even his contemporary deities don't posses casts some mystery on just how far back his story flows.
His knowldge is something that he is willing to share, and he acts as a teacher for many of his followers. He imparts not only occult wisdom, but also helps us understand how the worlds around us function. More interstingly is how Manannan seems to learn from us as well. Omnipotency is akin to stagnancy, and Manannan delights in gaining from us as just as much as he enjoys giving.