What is the purpose of adding powdered psyllium husk??

Psyllium is a water soluble fiber, and the main ingredient in a product called Metamucil. It can also help with lowering cholesterol, alleviating some discomfort in colon disorders, and help regulate blood sugar levels with diabetics. I imagine since the recipe was designed to be gluten and dairy free, it is added as another healthy ingredient that won't hurt anyone, and can be of benefit to many. It's not a critical add in ingredient that will make or break the recipe - just a nice thing to add in if you can or want to. Sneak in a little fiber - have a little of what you like, and something that's a little good for you at the same time kind of thing.

I see it's an optional ingredient, but I'm guessing it would be a good thing to add, if you can. Psyllium has thickening properties (rather dramatic, when mixed with liquids). Although I haven't made this cake, from the list of ingredients one would think that the psyllium will improve the texture of the crumb. That said, I've made almond flour cakes without psyllium which have turned out beautifully.
Hope this helps. ;o)