How do Orchid roots actually work?

[cythaenopsis]

I've seen photos of orchids in the wild, nestled in the crevices of tree trunks with their roots stretching out in a variety of directions. So when it rains, the water just rolls over those extended roots and falls away. I guess in most cases for a thriving orchid, debris from the trees (like moss) will collect in the crevices and absorb moisture, which will supply water for the roots there. And this is essentially what we're copying, by having our orchids potted with moss and other semi-porous filler. Correct?

So are some of these roots reaching out from the plant not serving to absorb water, but instead intending to help anchor the plant to its surroundings? The exposed roots above and outside the moss don't seem to serve any apparent purpose, with respect to taking in water. Or do they take moisture from the humid air?