As any aetherite is for, the aetherites in the Beast Tribe strongholds are a specific type of crystal that directs the flow of aether between two points, connecting them as a sort of direct gateway. People themselves can attune to aetherites to teleport between locations. The aetherites are not in and of themselves important to the existence of a primal, but instead simply connect to a secure location where the Primal is summoned, presumably because the great stores of crystals are there for the summoning and they don't want them stolen, or that the spot itself is holy to the Beast Tribe in some way. I believe Moenbryda gave the explanation during the Shiva questline as to their importance. It had a lot to do with the rivers of aether in the world and that without an aetherite acting as an anchor, one could simply dissipate in the flow of aether.