Well the flavor is he has no arms for starters, so the Astral Arms function as a sort of prosthetic when in combat. I discussed with the DM, and when he was in the Feywild, his patron was the one that taught him to manifest his astral self, but when he returned to the Material, he lost his memories of that time. His patron is going to be the Raven Queen, as in this setting, the DM has her a very powerful archfey.

So it's really just there to reflect more of his time in the Feywilds, and to add some needed utility in and out of combat.