Primal charm used to be for the Primal wand focus. I think that's changed in 1.7.10 because of stuff being moved to the Eldritch tab.Last I checked, didn't you need a Primal Charm in your inventory for some time to unlock the Warding focus? I recall having a message pop up about new knowledge after holding a Primal Charm in my inventory for some time - although I tend to keep a few Primal Charms on me anyway, as they tend to spew out free vis.