Based upon my own observations of plants in the wild, plus when drastically over potting them, I have concluded that aerial roots are "guy wires" that the plant is sending out in an effort to mechanically secure itself.
In nature, phals will have literally meters and meters of roots spread out on the tree limbs. If you consider the large, floppy leaves that grow from a relatively small base, wind can be a real problem. By sending out roots from a little higher up, those roots can anchor the plant, securing the "sails" from tearing it off the tree and dumping it on the ground.
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